I use Swift Mailer, for my mailing needs which can be quite demanding, and never had a problem with it..

When I'm doing mailing I tend to run the script from a cron job so it doesn't impact the webpage, as with each itteration or mail sent you can't leave the page so you are in limbo, whereas running a php script from a cron job every hour or whatever period you need it gets you away from that issue. As long as you have your database setup well, maybe add a field with a "mailsent" type bool, int set it 1 or 0 as required, then it sends to the required recipients dependant on a small query...

http://swiftmailer.org/

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Thanks... Simon