I've got Email Manager and it has some pretty good features but it has a lot of drawbacks especially given a large user base.  The problem I had with it was also emailing 5000+ users and those emails then going to my host which has this limit on how many emails I can send....so some emails get sent and some don't.  That may not be a problem with the Email Manager module but rather my host.  At any rate, the Email Manager has a setting that allows you to throttle how many emails are sent per millisecond but I've found it doesn't actually work.  I tried to set it so it only sent one email per 3 seconds or something to meet my host email restriction but Email Manager ignored it.  Email Manager controls are also all messed up if you use an XHTML standard for your DNN site.

Also, the guys that do Email Manager are absolutely horrible when it comes to support.  I mean horrible.  And the software is a pain in my opinion.

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I'm too poor for anything other than the community version