[SOLVED] How do you handle "550 No Such User"

[zolthar]

Currently I have an LDAP to an Exchange 2016 Server and currently quarantine non-existent users (eventually block).

However if someone accidently makes a mistake with the email, ideally it would be best they get a notification that no such user exists - however this gives rise to spammers and possibly directory harvests from what I have read.

I understand that it is supposed to be sent from the Exchange, however using Verify Recipients (Yes 450) seems to get a a few issues whereby most emails verifies correctly, and then a very small proportion gets blocked in a recent incident whereby an email from a real sender gets blocked. So I had to disable Verify Senders and the emails were delviered successfully.

But then if I enable notification to Non-LDAP via PMG:

1. How long will the reply email stay in PMG queue?
2. Could this be used for backscatter attacks?

How does everyone handle the No Such User event?

 

[Stoiko Ivanov]

However if someone accidently makes a mistake with the email, ideally it would be best they get a notification that no such user exists - however this gives rise to spammers and possibly directory harvests from what I have read.

PMG usually sends NDR messages to mails it receives on its internal port (default port 26), and does not sends them if they are received on the external port (for this check the description in the reference documentation (section 4.6.5 in https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#_mail_proxy_configuration)

How long will the reply email stay in PMG queue?

usually mails are queued in PMG for 5d (see http://www.postfix.org/postconf.5.html#bounce_queue_lifetime)

for questions regarding recipient verification - take a look at the corresponding postfix howto:
http://www.postfix.org/ADDRESS_VERIFICATION_README.html

I hope this helps to get a good idea how those parameters work inside PMG