I've just recently started supporting Mercury again, after several years in a different job. If my memory serves, we used to use mailing lists with large number of users and had no problems with SMTP server limitations on the number of addresses in the message. It seemed like Mercury was able to break the message up into separate envelopes or something. I always used SMTP-C 'cause we were on a slow dial-up.
I've got Mercury 4.61 running and a mailing list setup with a little over 100 members. I first tried SMTP-C to my website host's SMTP but still get the "too many recipients" message, so decided to try SMTP-E. There I get a lot of issues about bad connections or refused connections.
I'd rather use SMTP-C if I can get Mercury to break up the message rather than sending to everyone in one big chunk.
Any thoughts?