A simple thought-experiment can answer this for you.
Imagine that I run my own mail server for my domain and I don't implement SSL on it. No matter what protocol you use to talk to your mail server, it will have to use plain SMTP to talk to mine.
Since there exists a case where it is impossible for the next hop to use SSL, your suspicions are confirmed.
SSL for SMTP communications was designed to protect the password you use to authenticate to the mail server, not the content of the email.
Taking this a little further, since SSL is not a required part of the email system, even when there is a choice between using SSL and not using it, MTAs will generally choose to use plain SMTP because it's simpler and cheaper.
The final hop (from POP or IMAP server to client) can also be encrypted or not and that is also completely out of your control and completely independent of whether you used SSL to talk to your MTA.
As AviD said in the comments, use PGP, GPG or S/MIME to protect the contents of the email and use SSL to protect the password you use to authenticate to your mail server.