Ik weet niet wat er mis is met de SMTP-service

[satonotdead]

You need to check the security option because it isn’t very precise and you need to turn it off (without compromising your security).

Setting up SMTP

If you want to disable TLS, you need to uncomment this line, and set it to false: #DISCOURSE_SMTP_ENABLE_START_TLS: true # (optional, default true) However, there’s no relationship between POODLE and newer TLS protocol versions, nor is there any relationship between SMTP and POP3. Mailgun does support TLS on SMTP connections, they just don’t support it correctly.