On occassion and from a variety of networks and clients, Sent messages don't get saved.
I'm wondering if these log messages could be why:
May 3 14:16:39 mail.box postfix/smtpd[5195]: connect from 192.168.1.58
May 3 14:16:39 mail.box postfix/smtpd[5195]: SSL_accept error from 192.168.1.58: -1
May 3 14:16:39 mail.box postfix/smtpd[5195]: lost connection after CONNECT from 192.168.1.58
May 3 14:16:39 mail.box postfix/smtpd[5195]: disconnect from 192.168.1.58
May 2 22:14:30 mail.box dovecot: imap-login: SSL_accept() syscall failed: Connection reset by peer [::ffff:192.168.1.58]
May 3 14:16:39 mail.box postfix/smtpd[5195]: SSL_accept error from 192.168.1.58: -1