[Mailman-Users] Subscribe confirmation failure

[Chuck Baslock]

Had MailMan set up at a host(Plesk).

Been trying to get it running for several days.
Support techs have been no help.

postmaster at yyyyyyyy.zzz works.

Receive following error when replying to a subscribe request confirmation.
Can anyone decipher this and lead me in the right direction?

  ----- The following addresses had permanent fatal errors -----
<xxxxxxx-request at lists.yyyyyyyyy.zzz>
    (reason: 553 sorry, that domain isn't in my list of allowed rcpthosts
(#5.7.1))

   ----- Transcript of session follows -----
... while talking to lists.yyyyyyyy.zzz.:
>>> RCPT To:<xxxxxxx-request at lists.yyyyyyyy.zzz>
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
550 5.1.1 <xxxxxxx-request at lists.yyyyyyyy.zzz>... User unknown