The issue of newsletters is always one of interest. Whilst there are a number of extensions out there that will run newsletters in Joomla, the question of server compatibility is always looming.
The extensions available all effectively mean that the newsletters will be sent from the server where the Joomla site is hosted and this can create problems. If hosted on a VPS or dedicated server, these are reduced, but there is still the question of server load on a VPS at least where other users share the same space. Depending on the size of the list, mailings should always be staggered over a period of time either within the software or via a cron job.
If you have a site hosted on a shared hosting service, this becomes even more of a problem. Make sure you check the Terms of Service thoroughly to find out how many emails can be sent each hour. I found this out to my detriment over Christmas 2007 when I sent out a Merry Christmas message to all my regsitered users and got shut down for being a spammer! There is no "innocent until proven guilty" rule with hosting companies. It took me 3 days of emails back and forth to get the hosting reinstated!
Depending on the nature of your site and your user base, there is also the danger of users sending spam complaints to the hosting company which can also cause detrimental effects. Your IP, apart from being shut down by the hosting company, could also end up in the spam servers registrar from which it is difficult to get it removed. These vigilante sites can cause endless amounts of trouble by distributing legitimate email addresses to ISPs and you can find your email address listed in their spam addresses and no mail will get through to users.
These reasons are the ones that companies like Aweber and GetResponse trade on in terms of protecting your IP address because THEY send the emails and they also "claim" that their deliverability rates are enhanced because of their reputation with ISPs. It is possible to pipe user registrations through to services such as these to ensure double opt in.
Personally, I find this whole area quite frightening. I don't use Aweber or GetResponse, but another company where emails to my lists are sent from dedicated servers totally remote from my IP address.
I'd be most interested to hear of other people's personal experiences with mass email and how they protect themselves from spam complaints and their IP and hosting accoounts from being shut down.
Cheers,
Ric