[grandeur_69]

Your problem is usually related to server load. You probably need to increase the time between pages. This is the biggest problem I've had with this mod in the past, was that sending such big HTML emails really bogs down the server, especially if you have a virus scanner going through every email on sending. Also, the more bounces the server has to handle, the more it bogs down further.

Sending to active users is one solution. Increase the time between sends is another. Dedicated mail server would work, as well. I send to over 100k users every week and it never fails (sending from 4 forums at once, too), but I have a dedicated server.

I have added my own hack using .htaccess and some random image code to track openings. I will probably run some SQL in a month or so to remove users that haven't either opened the email or aren't active (in the last 300 days, for example). That should severely reduce the number of emails being sent.

Of course, another option, if you know someone who has the skills, is to add a bounce-handler.