Sometimes an email sent from Titan Mail may get deferred and will not reach the intended recipient.
Logs would look like -
| 2021-07-05 08:07:07 | remi.girardon@gadz.org | Outgoing | Deferred | (host themis.gorgu.net[178.132.22.108] said: 450 4.7.1 <remi.girardon@gadz.org>: Recipient address rejected: Greylisting in action, please come back later (in reply to RCPT TO command)) |
What is Greylisting?
Greylisting is a spam reduction technique that can be very effective. It works by temporarily rejecting from client machines that are unknown to the server's greylisting service.
If the client is standards-compliant, it will attempt to re-send its message after its initial failed smtp session, and your receiving mail server will accept it. The client is then added to a list of known clients, and will not be delayed in the future. This means that the first e-mail from an unknown client will be delayed, but subsequent ones will be processed right away.
Most spam mailers, on the other hand, do not re-send messages after failed smtp sessions. Thus, in theory, greylisting effectively blocks the majority of spammers.
The main intention of Greylisting is to make sure that the recipient's inbox is clean when an email is sent from a server unknown to him.
What to do in this case?
Simply ask the Titan Mail user to make sure that the sending IP is present in the senders domain's SPF record