I'm trying to setup mail notifications via stmp on an OpenProject docker instance. I'm using exactly the same settings as my other services (gitea, nextcloud, GitLab via host's sendmail), but they don't work when using them in OpenProject. Here's what I'm setting up:

• Email delivery method: smtp
• SMTP server: mail.gmx.net
• SMTP port: 587
• SMTP HELO domain: (empty)
• SMTP authentication: login
• SMTP username: my.account@gmx.de
• SMTP password: myPassword
• Automatically use STARTTLS if available: true
• Use SSL connection: false

Whenever I send a test mail. I'm getting An error occurred while sending mail (556-Requested action not taken: domain does not accept mail ). I tried to google this oen but didn't find something that fixed the problem. My other services can send mails with these settings just fine. Any idea what the issue could be?