I hope I find a helpful solution here.
Namely, I have been having a problem with mail merge for some time now, and as I update sending mail with its help, it becomes more and more responsible.
When I did this before, there was no problem with sending 500 emails, but when it happened, it was about 100 or sometimes it was interrupted.
I have been using it for several years and have not encountered a problem before. Perhaps you have any suggestions as to what might be wrong?