if the data passing through your system and the server is not encrypted, it can probably be bypassed.
i am not sure what happens if you use stuff like PGP in your email client.
It is best to assume that all unencrypted communication going out of your computer is being monitored or intercepted and stored somewhere (maybe forever).
If you use PGP with best practices (e.g. don't let the private keys get compromised, good passphrases, etc), then nobody except the sender and receiver can read the message. If the computer of the sender or the receiver has a backdoor or is compromised, that's another thing. I wish PGP or S/MIME would catch on.