It's a soft expire for 30 days and then functionality starts to reduce; if you choose to not renew you should run the Open Source Edition installer over your Network Edition install.
-30: Warn at admin console UI login, via e-mail from your server and sometimes our company.
-+0: Warn end users at ZCO, iSync, at Zimbra Mobile login if possible.
+30: ZCO, iSync, at Zimbra Mobile stop working altogether, warn at web-client login.
+30: You can no longer create new accounts regardless if the below max user count since the license overall has expired.
At one point we discussed the following, but I haven't checked status lately - your old mail certainly doesn't become inaccessible:
+60: Stop sending via SendMsgRequest but not Postfix SMTP (so could use IMAP/POP clients).
+60: Stop Convertd (attachment HTML conversion) and HSM (hierarchical storage management) will not migrate new stuff to your secondary store.
+90: Hot Backup may stop.
At the end of 30 days if you choose not to commit (though we certainly hope you do!) kick off a
zmbackup (if your cautious take an additional rsync or cp of /opt/zimbra and any linked directories) then run the installer for the
same version number of FOSS over your NE.
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