If you want to be able to read encrypted mail you have sent (in your sent-mail folder), you will have to encrypt outgoing mail with multiple keys, one of them being your own. At one point there was apparently a mutt option called pgp_encryptself, but it seems to no longer exist.
$ gpg --list-key email@example.com pub 1024D/72A6205E 2003-07-20 Kevin George <kevin@... .net> uid Kevin George <kevin@... .com> [...]
The ID is the number after the first slash - 72A6205E in this case.
default-key ABCD1234 encrypt-to ABCD1234
Beware though, that this change will encrypt everything to your own key, not just mails sent from Mutt.
If you want to encrypt only mail with your own public key as well as the recipient's public key, you need to customize the pgp_encrypt setting in your .muttrc. If you customized pgp_encrypt_only_command or pgp_encrypt_sign_command, you just want to add to them. If not, find them in /etc/Muttrc.
Now edit the pgp_encrypt_only_command and pgp_encrypt_sign_command lines of your .muttrc or add new ones based on the entries in /etc/Muttrc, and include an --encrypt-to ABCD1234 parameter in both, where ABCD1234, again, is to be replaced by the key ID from your key. Restart Mutt and try sending encrypted email to someone else, then try reading it in your sent-mail folder.
After upgrades to Mutt, make sure to check your /etc/Muttrc to see if any of the pgp_encrypt_X lines have changed. You should only edit the lines in .muttrc for that reason. Not making changes the central configuration also makes sense in case the system is (going to be) used by other users -- they will appreciate that their encrypted mail is not readable to you. ;-)