Hi Garrett, it is mailcow and has been running for 2 years without any problem. The smtp port is 587. I set the mail in passbolt up as follows:
Email provider → Other
Authentication method → Username & password
Username → myemail@mydomain
Password → mypassword
Advanced settings
SMTP host → mail.mydomain
Use TLS → Yes
Port → 587
SMTP client → (empty)
Sender configuration
Sender name → no-reply
Sender email → no-reply@mydomain