Rejection returns with code "500" (or "5xx") should be considered as hard fail and not retry.
Seems it's a Outlook side issue, not mail server. It handles the rejection "improperly".

Seems you didn't understand my previous reply.

Postfix already rejects the email with code 500 (or 5xx) with error message "Recipient address rejected: User unknown in virtual mailbox table", mail client applications like Outlook, Thunderbird, or webmail should stop trying/retrying to send email when they get 500/5xx code. Obviously Outlook still wants to re-send the email which is not right.

It's possible, but the tradeoff is "huge" (IMO).

Note: i didn't test it myself yet, but it should work.

In /etc/postfix/main.cf, find this:

# Recipient restrictions
smtpd_recipient_restrictions =
    reject_non_fqdn_recipient
    reject_unlisted_recipient
    check_policy_service inet:127.0.0.1:7777
    permit_mynetworks
    permit_sasl_authenticated
    reject_unauth_destination

Move rule "reject_unlisted_recipient" after "permit_sasl_authenticated":

# Recipient restrictions
smtpd_recipient_restrictions =
    reject_non_fqdn_recipient
    check_policy_service inet:127.0.0.1:7777
    permit_mynetworks
    permit_sasl_authenticated
    reject_unlisted_recipient
    reject_unauth_destination

Just a remind and warning: with default smtpd_recipient_restrictions setting, Postfix already correctly rejects email sent to non-existing (local) mail users, it works as expected, and it saves system resources used to go through other rules defined in smtpd_recipient_restrictions. For example, rule "check_policy_service inet:127.0.0.1:7777" will go through iRedAPD service, it will query SQL server for some checkings.

Again, this is an Outlook issue, not iRedMail server side configuration.

Hi Huangbin,

I further isolate this issue and found out that only ActiveSync is affected.

* If I use Outlook create a POP3 or IMAPS Profile, and send an email to any non-valid same domain email, the email will actually goes to the Sent Item and I receive a Bounce email from System Administrator saying the email is undeliverable. This is same for using mobile phones as well.

* If I use Outlook to have a Profile using ActiveSync, or mobile, then the email will stuck in Outbox instead of going to Sent Item and gives a Bounce email.

I do understand the receiver email is invalid within the same domain, but we want to achieve is sent it and bounce with a non delivery instead of just stucking at Outbox in Outlook or the Mobile phones. I tried different mail clients also behaving the same. Like mobile, I use GMail app, also same behaviour, Default Iphone and Android phone Email app also same behaviour.

Hence, I find it is something to do with the email system portion but couldn't find where is it.

hope you can advise.

thanks!

The reporter of the issue is another iRedMail user and i met him few times in person.
Unfortunately, SOGo team didn't realize it's a bug.