I thought they were going to remove the mailman module. It isn’t even remotely usable for mailman 3. Not sure which mailman you are trying to use but Mailman3 was kinda painful to install on Debain 11.
Mailman has nothing to do with your mail server. Mailman is not a mail server. It is a mailing list management tool.
And, it probably isn’t the kind of mailing list management tool you’re after (if you’re trying to send marketing emails, it is not the right tool for the job). It is for mailing lists in the traditional sense of discussion groups that operate over email. i.e. two way communication. It is not suitable for bulk outgoing mail.
Since you are on Debian 10, you can simply install the mailman package. Debian 10 has Mailman 2, which is the only version of Mailman the virtualmin-mailman module supports. Debian -- Details of package mailman in buster