I believe that error relates to overall connections to the server and not just your number of connections... it's actually an error message returned by the server. This can happen if a large number of users are all trying to send email at the same time through the same server. Rather than allowing itself to get overloaded, the server basically says I'll take this many and after that people have to wait... normally the situation should fix itself within a couple of seconds...
Oddly enough the error only occurs when I try to Send...never when I try to Receive emails.
Also, the problem does not clear itself up within a few seconds. On several occasions it's taken 5-15 minutes before the email goes out. I've been waiting over 5 minutes just now, repeatedly trying to send out an email to one of my customers.
FWIW, my email account is on the quasor server.
If this is an "overloaded server" problem, it represents another disturbing quality lapse in their systems (the recent account password screw-up being a real PITA). This problem has been going on all week