I see the error sometimes too.
I think it's because my listname + address wraps to two lines.
Can I tell LISTSERV / LSMTP to put the address in one line or do I just have to shorten the list name so it won't wrap to two lines?

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Error-for:  [log in to unmask]
Error-Code: 0
Error-Text: Mailer mailgate.kk.dk said: "503 Sender already specified"

...

 Sender:       Bibliofind Books For Sale Mailing List - Under US$100
              <[log in to unmask]>

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Error-for:  [log in to unmask]
Error-Code: 0
Error-Text: Mailer mailgate.kk.dk said: "503 Sender already specified"

...

 Sender:       Bibliofind Books For Sale Mailing List - U [Aaron West]   nder US$100
              <[log in to unmask]>