I'm using my Smartermail server to collect bounce messages. The incoming emails use a VERP plus style format user+bounce_unique_id@smdomain (with @smdomain being the Smartermail domain)

Smartermail is not delivering the bounce messages to user@smdomain. It is returning "550 No such user here" as it seems to be including the VERP string when attempting delivery.

2021.11.19] 12:39:50.295 [XX.XX.XX.XX][31179915] cmd: RCPT TO:<user+bounce_61972a9130b4a@smdomain.com> ORCPT=rfc822;user+2Bbounce_61972a9130b4a@smdomain.com
[2021.11.19] 12:39:50.295 [XX.XX.XX.XX][31179915] rsp: 550 <bounce+bounce_61972a9130b4a@smdomain.com> No such user here
[2021.11.19] 12:39:50.561 [XX.XX.XX.XX][31179915] cmd: RSET
[2021.11.19] 12:39:50.561 [XX.XX.XX.XX][31179915] rsp: 250 OK
[2021.11.19] 12:39:50.826 [XX.XX.XX.XX][31179915] cmd: QUIT
[2021.11.19] 12:39:50.826 [XX.XX.XX.XX][31179915] rsp: 221 Service closing transmission channel
[2021.11.19] 12:39:50.826 [XX.XX.XX.XX][31179915] disconnected at 19/11/2021 12:39:50 PM

From the Postfix MTA relay log (sending the bounce messages to Smartermail server):

postfix/smtp[185624]: A571A3F571: to=<user+bounce_6192143575613089061952@smdomain.com>, relay=mail.xxxxxxxx.com [XX.XX.XX.XX]:25, delay=0.02, delays=0.01/0.01/0/0, dsn=5.1.1, status=bounced (unknown user: "user")

Any ideas why this is occurring? Thanks!