SMTP Relay - (IPv4 Only) for a Specific Domain - M365 Connector?

p0se

Hi everyone,

I recently started using the relay feature in Mailcow for a specific domain. Emails from this domain are sent to a Microsoft 365 connector. While setting up the connector and the Mailcow Relay feature, I noticed that the connector on the Microsoft side only supports IPv4. I was able to verify in the logs that anything sent to the connector via IPv6 cannot be delivered.

To fix the problem, I simply disabled IPv6.
Now, all emails sent to the connector via the relay are delivered successfully.

However, I’d like to continue using both IPv4 and IPv6.

Is it possible to set a specific domain (the relay domain) to IPv4 only?
So that Mailcow can continue to provide all other features via IPv6?

p0se

based on the following two posts:

https://community.mailcow.email/d/3760-send-mail-only-using-ipv4/2

mailcow/mailcow-dockerized6877

I’ve updated the configuration file data/conf/postfix/extra.cf.

smtp_address_preference = ipv4
smtp_bind_address = [your_ipv4_address]

I’m not entirely satisfied with this yet.

Maybe someone else has an idea or a tip on how I can block IPv6 for a specific domain.

ETNyx

Hi, In theory you can make own “transport” by modifying master.cf. But there is chance this change will not survive MC update,..

So in theory master.cf

smtp_ipv4   unix  -       -       n       -       -       smtp
    -o inet_protocols=ipv4
## add your own configs for example special name for logs for better debugging,...

extra.cf

sender_dependent_default_transport_maps = pcre:/opt/postfix/conf/relaydomain_ipv4.pcre, proxy:mysql:/opt/postfix/conf/sql/mysql_sender_dependent_default_transport_maps.cf

data/conf/postfix/relaydomain_ipv4.pcre

/@relaydomain\.tld$/i   smtp_ipv4:[connector-host]:25