SMTPQ max threads?

Question: With the SMTPQ service, do you have some guidelines on the best practice
settings for the number of threads required for sending different quantities
of email?

Answer:
The SMTPQ is an SMTP client, and thus it connects to SMTP servers to send email. However, it may be configured to use any number of threads so theoretically each thread may be communicating with a different SMTP server. The SMTP server to be used for an email is determined at the time mailman.SendQ is called. The mailman.SmptHost property value, and other mailman property settings such as SMTP login/password, encryption/signing requirements, etc. are included with the email passed to SMTPQ.

To answer the question: If all of your emails are sent via the same SmtpHost, you wouldn’t want to overload that SMTP server by using too many SMTPQ threads. Also, some SMTP servers limit the number of simultaneous connections accepted from a single IP address, so if the SMTPQ thread count is too high, and they are all trying to send via the same SMTP server, some will be rejected.

This blog post provides more information about using multiple SMTP servers in a round-robin fashion:
SMTP Round-Robin