I have recently successfully managed to get a Postfix/Dovecot setup working with a RELAY (no credentials) provided by my ISP since they block all outbound traffic on port 25.
Question is how secure and private is this setup?
I have setup a test account in Thunderbird, which uses port 993 for pickup (SSL/TLS, Normal password) and port 587 for mail submission (STARTTLS, Normal password) from Thunderbird to my outgoing server. This seems secure so far, correct me if I'm wrong.
However, it seems to me that the use of a relay defeats the whole structure of privacy. Can my ISP see the contents my emails I send out and receive through this relay? If I switch to another host that does require credentials to use it (Eg. smtp.gmail.com), would Google be able to see the contents of my emails?
What would be the most private setup in this case?