Old, but not exist User in Active Directory

Question

old employees appear in the Mail Manager who no longer exist in the AD. They are still receiving emails from Sophos, although these mail addresses are no longer present in Exchange. Verified by the following Powershell script: 
 get-recipient -results unlimited | where {$_.emailaddresses -match " user@domain.com "} | select name,emailaddresses,recipienttype 
 Sophos will then try to deliver the mail to the old employee and Exchange will send a mail to do-not-reply@fw-notify.net 
 The error Message is "Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound; not found'" 
 Telnet : 
 MAIL FROM: existuser@***x.com 
 250 sender ok 
 RCPT TO: notexistuser@***.com 
 550 5.1.1 user unknown 
 I have also encountered the following link: 
 https://community.sophos.com/products/unified-threat-management/f/general-discussion/21138/quarantine-report-to-old-non-existent-email-users 
 Unfortunately I cannot find a solution here. Can anyone help me with that?

Mailaddress query in Exchange Management Shell

LuCar Toni · Answer

Do you use Callout as a Recipient Verification? 
 
 Then you need to delete the callout database. 
 
 The Callout Database can be found in the following directory: /var/storage/chroot-smtp/tmp/ram/db_input # ls -la 
 -rw-r--r-- 1 chroot chroot 12288 2012-03-19 11:41 callout 
 -rw-r--r-- 1 chroot chroot 0 2012-03-19 11:41 callout.lockfile 
 
 To flush the Cache delete the callout files, rm -f /var/storage/chroot-smtp/tmp/ram/db_input/callout*

LuCar Toni · Answer

Quite easy to answer. 
 You need a DB to stop flooding the Mail server. It would take to long, to "ask" everytime the Exchange, "is this mail address there or not?". 
 UTM is build to server X.000 Mails per Hour. If we would ask each time the Exchange, it would delay each mail in his SMTP Transmission a lot. 
 Cleanup process is quite nice idea but nobody made it. This DB is in UTM for ... Long long time.