Set up SMTP relay

Last change on 2026-08-28 • Created on 2026-08-28 • ID: MA-5F350

SMTP relay via a subdomain / Addon Domain

If you have migrated your mailboxes to Microsoft 365 while still using individual mailboxes on your managed server, you may encounter issues when sending emails.

This can occur when the mail system for the domain is managed by Microsoft 365, while applications or services on the managed server still need to send emails using addresses from the same domain.

In this scenario, the following workaround can be used to provide a separate SMTP relay configuration:

  1. Disable the mail system for the domain

First, disable the mail system for the domain <your-domain.tld> in konsoleH.

  1. Create a subdomain as an Addon Domain

Create an “Addon Domain” in the hosting account of the domain, for example:

relay.<your-domain.tld>

Configure this as an external domain.

  1. Configure the DNS records for the subdomain

For relay.<your-domain.tld>, create an A record, an AAAA record, and an MX record pointing to your managed server.

Use the following records as a template:

relay.<your-domain.tld>.    IN A    <IP of your server>
relay.<your-domain.tld>.    IN AAAA    <IPv6 of your server>
relay.<your-domain.tld>.    IN MX 10    <servername>.your-server.de.

Make sure that the IP addresses and server name are replaced with the actual values of your managed server.

  1. Create an SMTP mailbox

Create a dedicated mailbox for SMTP authentication under the newly created subdomain.

For example:

smtp@relay.<your-domain.tld>

This mailbox will be used to authenticate applications or services that need to send emails via the managed server.

  1. Configure DKIM and SPF

Enable DKIM for relay.<your-domain.tld> and configure the corresponding DKIM and SPF records.

In addition, update the SPF record for the domain <your-domain.tld> to authorize your managed server as a permitted sender.

For example:

a:<servername>.your-server.de

This allows the managed server to send emails using sender addresses such as:

noreply@<your-domain.tld>

Make sure that any existing SPF mechanisms are preserved when updating the SPF record. Avoid creating multiple SPF records for the same domain.

  1. Configure the SMTP connection

For SMTP-based email delivery, for example from a PHP application or another service, use the previously created mailbox for SMTP authentication:

smtp@relay.<your-domain.tld>

The actual sender address can still be an address belonging to the domain, for example:

noreply@<your-domain.tld>

This separates the SMTP authentication from the visible sender address. The application authenticates against the dedicated SMTP mailbox on the subdomain, while the email can still be sent using the desired sender address from the domain.

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