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]:........