Twin,
I have not seen CLI, general dial up modem connections typically have a usage restriction as to multiple logons to the same pop server (pop in this case being point of presence, not to be confused with a mail pop server).
I can think of one instance where this may have been in application is where no authorization may be in place, such as a dedicated server connection, we would call that number constantly looking to connect, some dedicated ISDN services also work that way.
We used to have people who would "shotgun" modem connections to get higher bandwith, and took that away, with the exception of dual channel ISDN, whose usernames were listed in a separate lookup table for those servers to allow dual channel logons.
However for mail, smtp severs typically go by IP address tables (the username and password you enter in a mail program is not for smtp, but for pop only), then if applicable they go to lists of acceptable sending domains.
For example, our hosting company allows our customers to send thru our SMTP, they could send out any mail thru them, however as they are not on our "network" (we are no longer an ISP) they must check the pop server of their hosted website account before they are allowed to send out, this confirms that they are acceptable. (I won't detail this, but that is how it is addressed in the shared hosting environ here.)
Cheers!