I have installed Subscribe2 plugin to Wordpress and working fine with it for 100s of members for a long time until some users complained me that they don't receive any email from us at all. I investigated the problems and found out that it was due to my hosting with GoDaddy that limits SMTP to 1000 email recipients per day.
GoDaddy even doesn't support external SMTPs. They have completely blocked SSL/TLS ports so great email services like Gmail isn't possible with them (Gmail is also having a limit of 500 emails per day with each email supporting up to 100 recipients. i.e.: 500x100 = 50000 which is more than enough for any user). The GoDaddy support guy called Adma Gomez confirmed that. He is really a great support guy. He explained all these very clearly. He said it is a GoDaddy policy to control spam with such limitations.
I was totally upset and decided to change the hosting to another server. But every hosting company I contacted had such limitations. Most of the people are giving 250 relays (recipients per day). There were some hosting companies who support external SMTPs including some free hosting companies (I have bad expereince with free hosting so I didn't want to even think about it). When you buy a hosting account, consider external SMTP support as well.
There wasn't any built in way to throttle emails with WordPress unless we write something by our own.
Luckily, thankful to the author of Subscribe2, I found his email queue solution that we can control the number of emails per day. Though it is not free, it is a very nice job by him. Have a look at http://wpplugins.com/plugin/76/wordpress-mail-queue-wpmq/