I had a local postfix mta running under Snow Leopard for testing purposes. After updating to Lion, postfix stopped working.
A good starting point is to run:
tail -f /var/log/mail.log
The first fatal entries in the log
postfix/smtpd: fatal: open /etc/postfix/submit.cred: No such file or directory
postfix/master: warning: process /usr/libexec/postfix/smtpd pid 1001 exit status 1
postfix/master: warning: /usr/libexec/postfix/smtpd: bad command startup — throttling
Which brought me to this in the Apple support forum.
After fixing that the next error on the log was
valid hostname or network address required
No further hint where the hostname or network address is missed. After playing around with hostnames and inet_interfaces I thought if something changed in the network in general. Bingo! IPv6 support!
Changed inet_protocols = all to inet_protocols = ipv4 in the main.cf and the postfix mta started as supposed.