Is there a way to mass email lets say, 100 people with a single connection but for the email to be different for each user. I need small variables changed like their name, and an authentication key to auto log them into the site when they click links.
Or is it really not that bad to be establishing that many connections to the mail server?
Edit: It's not that bad even if it's 10k members?
Short answer - it is really not that bad. The cost of connection for an SMTP session is quite low.
It is possible to send multiple emails serially using the same SMTP session, but that will involve writing your own mailserver access code. I suspect you won't gain much for the effort and may as well just call your language's mail() function repeatedly.
Some mailservers will only allow connections at a certain rate (especially if they are concurrent connections) but otherwise there isn't any real problems with it.
How about http://postageapp.com? It looks like they have message variables that let you change the name for everyone.