Cant send mail via smarthost with custom port

[Sysadminfromhell]

What's this issue about?

Sending mail via smarthost with custom port isnt working

Expected behavior

Send E-Mail via Smarthost

Actual behavior

The connection gets closed by the "Oh my form" host after STARTTLS:

Apr 17 10:04:31 mx postfix/smtpd[218101]: connect from unknown[172.16.24.123]
Apr 17 10:04:31 mx postfix/smtpd[218101]: SSL_accept error from unknown[172.16.24.123]: lost connection
Apr 17 10:04:31 mx postfix/smtpd[218101]: lost connection after STARTTLS from unknown[172.16.24.123]
Apr 17 10:04:31 mx postfix/smtpd[218101]: disconnect from unknown[172.16.24.123] ehlo=1 starttls=0/1 commands=1/2

Settings are:
MAILER_URI: smtp://10.0.1.3:26

Steps to reproduce

Set a host with "custom" smtp port.
Try to send mail (Register a User)

Environment

Debian 11
SmartHost (Proxmox Mailgateway) for internal sending via Port 26