We have used "Mail Merge toolkit" (mapilab.com) for some time now with Outlook 2003 / Windows XP to send about 400 emails each week.
Recently, we find that when we send several hundred emails, only the first 95-100 go through properly.
The remaining emails are missing the graphics elements. They have 'placeholder' icons instead. All the text gets through.
If you look at the emails in the 'sent' folder, there is a clear size difference. The first 75-100 are 96KB, and the rest are 3KB.
The mapilab tech suggested it's a outlook limitation / the OLK temporary folder is filled / etc. Clearing this folder has no effect.
There does appear to be some Outlook related limit in the 99-100 email range. But it also appears to be something that changed since May -- we have used it with no problems for 3 years before that.