On 28-04-2021 22:41, Wietse Venema wrote:
Asai:
Greetings,
We are getting reports, more and more, of email clients (Type App, Mac
Mail, iOS Mail) that seem to send email, and show that the email has
been sent on the client, but the mail server has no record of email ever
reaching the SMTP service, nor does it even seem that the client is
connecting to the server.
This means that client systems are compromised with malware
that sends email directly to the outside world, bypassing
your mail server.
To stop these, block outbound port 25 on your firewall for all
systems except your mail server.
The OP has not specified whether the intended recipient ever receives
the message, only that the sender thinks it's sent, and that the
expected MSA doesn't see the message in transfer.
In the past, I've also seen desktop malware scanners that setup a local
SMTP proxy for all outgoing email. It might be that the Mail client
sends the message successfully to the proxy, but the proxy sends it to
/dev/null and doesn't inform the desktop user properly.
Kind regards,
Tom
