Document how to test that outgoing port 25 connections work

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There is a thread about it on the forum: https://support.delta.chat/t/secure-join-between-accounts-on-separate-chatmail-servers/2932

Hetzner has FAQ entry: https://docs.hetzner.com/cloud/servers/faq/#why-can-i-not-send-any-mails-from-my-server

DigitalOcean documentation: https://docs.digitalocean.com/support/why-is-smtp-blocked/