Just relay it out via a trusted host, in addition to setting up DKIM, SPF, etc.; that solved all the problems for me. My email server is working excellent for many years now (OpenBSD, OpenSMTPD, dovecot) with Rainloop as web-based frontend.
Taken from my smtpd.conf:
table secrets { mylogin = [email]@ziggo.nl:[pass] }
accept tagged DKIM for any relay via tls+auth://mylogin@smtp.ziggo.nl:587 auth <secrets>It kind of defeats the purpose, since now the third-party will be able to read all my outgoing emails, and have control over them.
Relaying via the ISP's mail server though (even with TLS) seems like it would disclose the message contents.