Mailings cannot have attachments.

Individual messages from mailings are not stored in the database, so when the mailing is finished and purged, there is no way to view the outgoing messages it generated.

There is no limit to the number of recipients, but there is a unique sender for a particular mailing, and it has to be declared as a sender identity in File->Preferences->Identities.

Aside from the subject line, there is currently no way to provide additional header lines.