IONOS Mail Probleme: Trouble Sending Emails — How to Fix It (2026)

IONOS confirms that sending failures may result from incorrect SMTP settings, authentication errors, blocked ports, rejected sender addresses, or bounced messages.

Its official SMTP server for Mail Basic and Mail Business is smtp.ionos.com, using port 465 with SSL/TLS or 587 with STARTTLS.

Updated: October 30, 2026 | Service: IONOS Mail Basic and Mail Business | Problem: Outgoing emails fail, remain in Outbox, or bounce back

Why Is My IONOS Email Not Sending?

IONOS provides professional email hosting for businesses, website owners and individual users. You can access a mailbox through IONOS Webmail or connect it to an external email application such as Microsoft Outlook, Mozilla Thunderbird or Apple Mail.

When outgoing email stops working, the cause is not always the same.

Some users receive an authentication error immediately after clicking Send. Others see emails stuck in their Outbox, receive a delivery failure notification, or discover that messages are being rejected by the recipient’s mail server.

The difference matters because each symptom points to a different stage of the email delivery process.

For example, if your email application cannot connect to IONOS SMTP, the message may never leave your computer. If IONOS accepts the message but the receiving server rejects it, the problem involves delivery rather than the initial sending connection.

Before changing multiple settings, identify the exact error message and determine whether the problem occurs in Webmail, an external email application, or both.

IONOS Mail Problems: Quick Diagnosis Table

Fix 1: Verify the Correct IONOS SMTP Settings

Incorrect SMTP configuration is one of the first things to investigate when an IONOS mailbox can receive messages but cannot send them.

SMTP stands for Simple Mail Transfer Protocol. It is the protocol used by email applications to submit outgoing messages.

IONOS publishes the following server configuration for Mail Basic and Mail Business accounts.

Make sure the port and encryption method match. For example, do not select SSL/TLS on port 587 if your application expects STARTTLS.

If port 465 cannot establish a connection, IONOS recommends trying port 587 with the corresponding encryption settings.

Do not disable encryption as a workaround. IONOS requires modern encrypted connections for its email services.

Source: Official IONOS SMTP, IMAP and POP3 Settings.

Fix 2: Check Your Email Address and Password

IONOS requires authentication before an email program can send messages through its outgoing mail server.

A common mistake is entering only the mailbox name instead of the full email address.

For example, if your address is support@example.com, the SMTP username should normally be the complete address, not simply “support”.

Another frequent problem occurs after a mailbox password has been changed in the IONOS account but the old password remains saved in Outlook or a mobile email app.

To investigate:

1. Open the email account settings in your email application.
2. Locate the outgoing SMTP username.
3. Confirm that the full IONOS email address is entered.
4. Check that SMTP authentication is enabled.
5. Enter the current password assigned to the email mailbox.
6. Save the settings.
7. Send a short test message to an address you control.

Remember that your IONOS customer account password and the password assigned to a specific email mailbox may be different.

If you cannot remember the mailbox password, follow IONOS’s official password reset procedure.

Do not repeatedly guess passwords if the service is rejecting authentication attempts.

Fix 3: Test Sending an Email Through IONOS Webmail

Testing Webmail is one of the most useful diagnostic steps because it separates problems in your email application from possible mailbox or service issues.

1. Open IONOS Webmail.
2. Sign in using the affected mailbox credentials.
3. Compose a new message.
4. Send it to another email address you own.
5. Check whether the message is sent successfully.
6. Verify that the recipient actually receives it.

If Webmail Sends Successfully

This suggests that your mailbox can send through the Webmail interface. Focus on the settings or connectivity of the external email application that is failing.

Check the SMTP server, port, encryption, username and password.

If Webmail Also Fails

The problem may involve your mailbox, the message itself, an account restriction or a wider IONOS service issue.

Record the exact error message, check the official service status and contact IONOS support if the problem continues.

A successful Webmail test does not guarantee that every recipient will accept your messages. Delivery rejection is a separate issue.

Fix 4: Fix IONOS Emails Not Sending in Microsoft Outlook

Outlook users may find that incoming emails work normally while outgoing messages remain in the Outbox.

IONOS documents a specific SMTP authentication requirement for classic Outlook versions such as Outlook 2016 and 2019.

For those versions:

1. Open Microsoft Outlook.
2. Go to File > Account Settings.
3. Open the settings for your IONOS email account.
4. Select More Settings.
5. Open the Outgoing Server tab.
6. Enable My outgoing server (SMTP) requires authentication.
7. Select Use same settings as my incoming mail server.
8. Open the Advanced tab.
9. Confirm that the SMTP port and encryption settings match IONOS requirements.
10. Save the settings and send a test email.

Also check whether Secure Password Authentication (SPA) is enabled. IONOS advises disabling the SPA option for the POP/IMAP configuration covered by its classic Outlook setup guidance.

The new Outlook interface and Microsoft 365 versions may use different menus. Use the account configuration options available in your version rather than assuming every screen looks identical.

Official IONOS Outlook Troubleshooting Instructions.

Fix 5: Fix IONOS SMTP Errors in Thunderbird

Mozilla Thunderbird is another popular email application used with IONOS-hosted mailboxes.

If receiving works but sending fails, check Thunderbird’s outgoing server configuration.

IONOS recommends these values when Thunderbird displays an SMTP sending error:

To check the settings, open Thunderbird’s Account Settings and locate the Outgoing Server (SMTP) section.

Select the SMTP entry associated with the affected IONOS mailbox and compare its values with the table above.

Save your changes and test sending again.

IONOS also supports port 465 with SSL/TLS, but do not mix the port and encryption combinations.

If Thunderbird still cannot connect, test the same mailbox through Webmail to determine whether the issue is specific to Thunderbird.

Fix 6: Troubleshoot IONOS Email on iPhone, Android and Apple Mail

Mobile email applications can stop sending messages after an account password change, operating system update or incorrect server configuration.

Start by checking whether the device has a working internet connection.

Then review the outgoing mail server settings in your email app.

For IONOS Mail Basic and Mail Business, the server should be smtp.ionos.com with authentication enabled.

Use port 465 with SSL/TLS or port 587 with STARTTLS where supported by the application.

Make sure the email application is using the correct mailbox password.

On iPhone and iPad, the location of account settings depends on the iOS version. On Android, it depends on whether you use Gmail, Samsung Email or another mail application.

For Apple Mail on macOS, review the account’s outgoing server settings and confirm that the selected SMTP account belongs to the correct mailbox.

Do not remove an account from a device before confirming that its existing mail is safely stored on the server or backed up. This is particularly important for accounts configured with POP3.

IONOS Email Setup Guides for Desktop and Mobile Devices.

Fix 7: Try SMTP Port 587 if Port 465 Is Blocked

Some networks restrict connections to certain outgoing email ports.

IONOS explicitly states that if an email program cannot connect through port 465 despite correct settings, port 587 is available as an alternative.

To try the alternative configuration:

1. Open your email application’s outgoing server settings.
2. Confirm the server is smtp.ionos.com.
3. Change the SMTP port from 465 to 587.
4. Change the encryption setting to STARTTLS or the equivalent option documented for your email application.
5. Keep SMTP authentication enabled.
6. Save the settings.
7. Send a test message.

Do not switch to an unencrypted connection simply to bypass a network restriction.

If neither port works, check whether your firewall, security software or network administrator is blocking the connection.

On a company-managed device, ask the IT administrator before changing security settings.

Fix 8: Check IONOS Sender Address Restrictions

One important IONOS policy can explain why an email account suddenly stops sending messages when a different From address is selected.

Since January 29, 2024, IONOS Mail Basic and Mail Business SMTP servers have restricted sending with alternative sender addresses that use a different domain from the authenticated email account.

For example, if you authenticate as hello@example.com, attempting to send with a From address at a completely different domain may be rejected.

This can affect email applications, website scripts and systems that relay mail through IONOS.

To troubleshoot:

1. Identify the mailbox used for SMTP authentication.
2. Check the From address on the message.
3. Compare the sender domain with the authenticated mailbox’s domain.
4. Review any aliases or alternate sender identities.
5. Correct the configuration to comply with IONOS’s sender-address policy.

Do not assume that an address is permitted simply because your email application allows you to enter it.

Official IONOS Sender Address Policy.

Fix 9: Understand Bounce Messages and SMTP Error Codes

Sometimes your email application reports that a message was sent, but a delivery failure notification arrives afterward.

This is different from a local SMTP connection failure.

A bounce message usually contains a numerical SMTP status code and an explanation from the mail system that rejected the message.

Examples include temporary delivery failures, invalid recipient addresses, policy restrictions and rate limits.

The same three-digit code can have different explanations depending on the receiving server and the full response text.

Do not diagnose an email failure from the number alone.

IONOS publishes separate guidance for messages rejected by IONOS-hosted recipients and messages sent from IONOS accounts to other providers.

IONOS SMTP Error Messages.

Fix 10: Check Whether IONOS Is Experiencing an Outage

If multiple IONOS mailboxes suddenly stop sending at the same time, a wider service issue is worth investigating.

IONOS recommends checking its status page when sending problems remain unresolved.

Look for incidents involving email, Webmail or mail server availability.

Check the date, affected services and region before assuming an announcement explains your problem.

If there is a confirmed incident, wait for IONOS to publish an update rather than repeatedly changing working account settings.

If no relevant outage is reported, continue troubleshooting the individual mailbox or contact support.

Important: This article does not claim that IONOS is experiencing a live outage on October 30, 2026.

IONOS Mail Receives Emails but Cannot Send: What Does It Mean?

Receiving and sending use different protocols.

IONOS generally uses IMAP or POP3 for receiving mail and SMTP for sending it.

That means your inbox may synchronize successfully while the outgoing mail server fails authentication or cannot be reached.

For reference, the standard IONOS Mail Basic and Mail Business incoming settings are:

If incoming mail works but sending fails, focus on the SMTP configuration first. Changing your IMAP server is unlikely to fix an outgoing authentication error.

Why Are My IONOS Emails Going to Spam?

An email that reaches a recipient’s spam folder has been delivered, even though the recipient may not see it in their inbox.

This is a deliverability problem rather than a failure to connect to the outgoing SMTP server.

For custom-domain email, check that your domain’s SPF and DKIM configurations follow IONOS guidance. DMARC can also help domain owners establish a policy for authenticated email.

Other factors include message content, sending reputation, recipient filtering and whether your messages are being sent to valid, consenting recipients.

Do not add arbitrary SPF records copied from another website. Use the records appropriate to your actual mail provider and sending services.

For persistent delivery problems, examine the complete bounce message or ask the recipient to check their spam folder.

IONOS Mail Problems After Changing a Password

If you recently changed the password of your IONOS mailbox, update the saved credentials in every email application that uses that mailbox.

For example, you may have updated Outlook on your desktop but forgotten that the same email account is configured on your phone.

Both applications may continue attempting to authenticate with different saved credentials.

Open each email application’s account settings and update the password through the official interface.

Afterward, send a test message from one device at a time.

If authentication still fails, use Webmail to confirm whether the new mailbox password works.

IONOS Mail Not Sending From WordPress or a Website

Website owners may encounter IONOS sending problems when contact forms, order confirmations or notification emails fail to arrive.

These failures can involve PHP mail configuration, SMTP authentication, sender-address restrictions or domain authentication.

If you use WordPress, confirm which mail transport your website actually uses.

For an SMTP-based setup, verify that the plugin is configured with the correct mail service and permitted sender address.

Do not assume that the standard IONOS Mail Basic SMTP settings apply to a third-party transactional email provider or Hosted Microsoft Exchange account.

Review IONOS’s official PHP mailer and SMTP guidance before modifying your website’s email delivery configuration.

For a business-critical website, test both the SMTP connection and actual delivery to a recipient mailbox.

When Should You Contact IONOS Support?

Contact IONOS if your mailbox cannot send through Webmail, correct SMTP settings still fail, or you receive repeated rejection messages that you cannot resolve.

Before contacting support, collect the following information:

• The affected email address, without sharing its password.
• The email application and version.
• The outgoing server and port configured.
• The exact error message or SMTP response.
• The approximate time of the failed send.
• Whether Webmail can send successfully.
• Whether the issue affects one mailbox or several.
• Whether you recently changed passwords, domains or sender addresses.

For sending and receiving problems involving IONOS mail servers, the IONOS Postmaster portal also provides a route for reporting mail-related issues.

Use the official IONOS Help Center for account-specific support.

Official IONOS Email Troubleshooting Resources

• IONOS — Unable to Send Emails
• IONOS — IMAP, POP3 and SMTP Server Settings
• IONOS — Troubleshooting Email Program Setup
• IONOS — Sender Address Restrictions
• IONOS — SMTP Error Messages
• IONOS Postmaster Support

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Editorial note: Add direct internal links to existing, relevant email troubleshooting articles after verifying their published URLs.

Editorial note: Updated October 30, 2026, using official IONOS email documentation. This guide covers IONOS Mail Basic and Mail Business. Hosted Microsoft Exchange and third-party mail services may require different settings. Seminarsonly.com is independent and is not affiliated with IONOS.