SMTP timeout with Zoho and Mailjet (except at start)

Describe the bug

Watchtower times out when trying to shoutrr via smtp. {"error":"failed to send using smtp: timed out","index":"0","level":"error","msg":"Failed to send shoutrrr notification","notify":"no","service":"smtp","time":"2024-05-27T00:09:21+02:00"}

At first I thought it was an issue with Zoho Mail , so I set up a Mailjet account for the SMTP relay. The behaviour is exactly the same. I tried setting up the SMTP parameters both as separate environment variables and as a single URI. No difference.

What is odd is that when watchtower starts, it succeeds in sending the email ! It's only subsequent emails that always time out.

Steps to reproduce

Setup SMTP notifications through Zoho or Mailjet. Start watchtower : receive the start email. Have watchtower update containers : no email received, log shows timeout talking SMTP.

Expected behavior

Receive an email everytime watchtover wants to send one.

Screenshots

No response

Environment

Synology DSM 7.2.1-69057 Docker version 20.10.23, build 876964a Docker Compose version v2.9.0-6413-g38f6acd

Your logs

{"error":"failed to send using smtp: timed out","index":"0","level":"error","msg":"Failed to send shoutrrr notification","notify":"no","service":"smtp","time":"2024-05-27T00:09:21+02:00"}

Additional context

No response

containrrr / watchtower