I sent you, via eMail, a modified version of your SendAttachments.aspx and SendAttachments.aspx.vb. If you did not receive the eMail please let me know where to send it.
See Page_Load() in .vb and you'll find the single line of code Session("...") that causes the problem.
It doesn't matter where the Session State variable is set - any Class or any Module.
If no Session variables are used, the Pause and Show Status buttons work as expected.
Feel free to press 'Show Status' at any time, and you'll get an update of the status Mail Merge.
You may press 'Pause' and the Mail Merge will be paused after the next message has been sent.
If Session variables are assigned, however, the postBacks are stacked until after SendMailMerge() completes.
Please note: I set a batch interval of 5000 and batch size of 1 to make it easier to pause during the Mail Merge.
I hope this describes the problem in adequate detail so this issue may be resolved as quickly and easily as possible.