We have an existing Zimbra server (v7) that has been in place for years. We're now receiving errors when sending to @gmail.com users:
***EMAIL REMOVED***: host gmail-smtp-in.l.google.com[18.104.22.168] said:
530 5.7.0 Must issue a STARTTLS command first. lp5sm2119372igb.1 - gsmtp
(in reply to MAIL FROM command)
Zimbra.log wasn't useful. It showed the message coming through and this same error message.
I don't understand what has caused this to change today. Nothing has changed on our end. No email is flowing to @gmail.com from us right now and users are getting bounce-back messages.
Any ideas on what may cause this and how I can resolve it?