To hammer this point home:

The sender and the receiver both have a copy of the email. The receiver's MTA pretty much has to have a copy of the email for sane antispam to work. Email itself intrinsically leaks the sender, recipient, and time as part of the SMTP message, which means anyone with access to email server logs can recover this information no matter how much security and encryption you try to put on top of it, and that means that whatever three-letter agency you care about probably has access to all the information they want anyways.

The only meaningful difference in security is going to be whether or not the outgoing MTA is going to try to use (or even require) TLS on the MTA-MTA connection, and what level of encryption is applied to the at-rest email.

Personally, I'm not too fussed about a mail system administrator being able to read my email because of the BOFH principle... it's not worth the sysadmin's time to read my email, because it's just not interesting.