# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
smtp      inet  n       -       -       -       -       smtpd -o content_filter=checknumber:dummy
checknumber unix -      n       n       -       -       pipe
   flags=Rq user=test-mail argv=/var/mail/hooks/check-number.sh -from ${sender} -to ${recipient}

virtual_alias_domains = example.com
virtual_alias_maps = hash:/etc/postfix/forwarding

server@example.com kyle@otherserver.com

Jan 11 22:42:15 s18324533 postfix/smtpd[8607]: connect from mail-sn1nam02on0061.outbound.protection.outlook.com[104.47.36.61]
Jan 11 22:42:15 s18324533 postfix/smtpd[8607]: BEEFE24748216: client=mail-sn1nam02on0061.outbound.protection.outlook.com[104.47.36.61]
Jan 11 22:42:15 s18324533 postfix/cleanup[8610]: BEEFE24748216: message-id=<fc8a712098e64dcfb85ced1b4f17f863@its-XXX>
Jan 11 22:42:15 s18324533 postfix/qmgr[8574]: BEEFE24748216: from=<kyle@otherserver.com>, size=13713, nrcpt=1 (queue active)
Jan 11 22:42:16 s18324533 postfix/pipe[8611]: BEEFE24748216: to=<kyle@otherserver.com>, orig_to=<server@example.com>, relay=checknumber, delay=0.22, delays=0.21/0/0/0.01, dsn=5.5.0, status=bounced (remote error in protocol. Command output: Email received (-from kyle@otherserver.com -to server@example.com) )
Jan 11 22:42:16 s18324533 postfix/qmgr[8574]: BEEFE24748216: removed
Jan 11 22:42:16 s18324533 postfix/smtpd[8607]: disconnect from mail-sn1nam02on0061.outbound.protection.outlook.com[104.47.36.61]

Jan 11 22:43:31 s18324533 postfix/smtpd[8607]: connect from mail-bn3nam01on0089.outbound.protection.outlook.com[104.47.33.89]
Jan 11 22:43:32 s18324533 postfix/smtpd[8607]: NOQUEUE: reject: RCPT from mail-bn3nam01on0089.outbound.protection.outlook.com[104.47.33.89]: 550 5.1.1 <1234@example.com>: Recipient address rejected: User unknown in virtual alias table; from=<kyle@otherserver.com> to=<1234@example.com> proto=ESMTP helo=<NAM01-BN3-obe.outbound.protection.outlook.com>
Jan 11 22:43:32 s18324533 postfix/smtpd[8607]: disconnect from mail-bn3nam01on0089.outbound.protection.outlook.com[104.47.33.89]