Can't "Send" reply - Only Mass Mail

My wife received an email with a couple of recipients in addition to her. She selected “Reply” and typed a response, added herself as a recipient (to verify that the email was sent) and put me in as a BCC.

When she pressed “Send” a little box dropped down giving her a choice of “Mass Mail” or some other option that implied that she would abandon the reply. Of course, choosing :Mass Mail" asked her to get a Pro subscription.

Why did emClient insist that she had to use Mass Mail to reply in the manner she did?

What other info would anyone like in order to help solve the problem?

Thanks,

Jamie
Petersburg, VA

Did she click SEND or did she click the down arrow next to SEND?