Up to a certain size of distribution list, pmail appears to copy the list
into the To: field. Above that size, it puts the user-given listname in the
To: field of the actual outgoing e-mail. E.g, if the list was called "Board
Members", the To: field in the actual outgoing e-mail would be

To: Board Members

(and the actual list of recipients would, of course, be in Envelope-To

Unfortunately, this violates the RFC-specified syntax for the To: field. The
display-name annotation needs to be followed by a colon and a semicolon.

Don't forget, even if your outgoing mailer is lax about the syntax, the
recipient's might not be so accommodating.

To circumvent the problem, make sure that the user-given name to the
distribution list ends with a colon and a semicolon.

The formal syntax for a group is:

group = display-name ":" [mailbox-list / CFWS] ";" [CFWS]

This is commonly seen when e-mail is sent to a bcc list and I believe both
the ":" and ";" are significant in the To header below:

To: undisclosed-recipients: ;

CJC