Sending mail using SMTPs fails because of allegedly missing auth scheme - despite them being present

[yogo1212]

Description

Twisted reports: twisted.mail._except.AUTHRequiredError: 502 Server does not support Client Authentication schemes [CRAM-MD5, LOGIN, PLAIN]

The mail server advertises these options when connecting with openssl s_client -connect mail.domain.io:465 (and sending EHLO):

250-mail.domain.io
250-PIPELINING
250-SIZE 512000000
250-ETRN
250-AUTH PLAIN LOGIN
250-AUTH=PLAIN LOGIN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250 CHUNKING

This is the relevant excerpt from the homeserver config:

email:
  smtp_host: mail.domain.io
  smtp_port: 465
  force_tls: true
  smtp_user: [email protected]
  smpt_pass: redacted

Possibly related to:
#15126

Steps to reproduce

• open app.element.io in a new private window
• enter homeserver (will disclose if required)
• press 'forgot password?'
• enter any email address
• press 'send email'

Homeserver

private matrix server

Synapse Version

1.121.1

Installation Method

Docker (matrixdotorg/synapse)

Database

postgres from the get-go

Workers

Single process

Platform

rocky linux, docker container

Configuration

see above

Relevant log output

see above

Anything else that would be useful to know?

No response