Okay, I believe I have noticed a bug in the Mail Users section of the core of xoops.

I have over 967 members. I selected the SEND TO Private Message box.

When I clicked to send the members a PM message, part of the normal program is it stops at each 100 sent and requires you to click a NEXT button.

HERE IS THE POTENTIAL PROBLEM

I noticed that the way it sends changed from Private Message in the list of the user names sent to just Mail.
I also believe this happened when I tried to use the EMAIL option too.
It appears to somehow reset or loose the checked option when you go to the next group of users.

I hope that makes since and can be checked or tested.