If you’re observing errors on your Zimbra servers’ mail-log similar to the one below, and your users are reporting password prompts on ‘sending’ e-mails.
Nov 24 17:12:07 areto postfix/smtpd[5528]: warning: SASL authentication failure: cannot connect to saslauthd server: Connection refused
Run the following commands under your servers root user.
ln -s /var/spool/postfix/var/run/saslauthd /var/run/saslauthd
zmcontrol restart

After Zimbra finishes restarting all the services, get your users to re-try sending mail.