RE: lmtp Fatal: setresgid Operation not permitted

I am getting the following error when Postfix attempts LMTP delivery :

"lmtp(REDACTED)<32674><7Jm0BSVopmaifwAAbW4UVQ>: Fatal: setresgid(121(mailbox_user),121(mailbox_user),8(mail)) failed with euid=111(mailbox_user): Operation not permitted"

mailbox_user is uid 111, gid 121 on my system.

Not related to low uid/gid numbers? I think newer OS have their limits increased to above 1000 or so.