SMTP ports encode the mail role

SMTP ports are not random provider settings.

They describe the role of the connection. When an app, backend, Listmonk, PocketBase, or automation tool sends through an ESP, it is submitting mail.

Start with 587 and STARTTLS, because that is the normal secure submission path. Use 465 only when the provider expects implicit TLS, and 2525 when standard ports are blocked.

Port 25 is a different job: server-to-server relay. It is often blocked, abused, and not the right first choice for application sending.

The useful habit is to configure SMTP by role first, provider second.

Related:

• SMTP and e-mail stuff
• Email APIs are product infrastructure
• Listmonk Docker config
• PocketBase Docker config
• Mailtrap ESP notebook