Correspondents receiving my mails as attachments...?

Your emails are coming thru to me in a different format – they come thru as an attachment and then don’t show on my reply…

A few of my regular correspondents have reported my messages arriving with blank bodies - but an attachment with the message body. And not every message - just some!

Has anyone discovered the reason for this? Is the issue at my end or theirs?

I try to ONLY correspond with Mac users, but occasionally, unknowingly find myself in conversation with a Windows user... could be that?