mac mail gremlin

[Hedwig]

I have a weird problem that continues to bedevil my Mac mail. An email message will appear in my in box and when I open it, the contents are completely replaced with an old piece of mail I previously received several months or a year ago--typically a piece that was read and discarded, such as a Borders sales announcement, a political announcement, a Facebook notification, etc. It is frustrating because if I can tell from the subject/sender that it was something I wanted to read, I have to try to contact them and ask them to resend it. I don't know where the discarded emails are being revived from, what is happening to the original contents of the messed-up mail, or how to prevent it. Quite often if it happens once in a day it will happen again. The old replacement emails are usually from a chronological sequence, for example, several days within a couple weeks' period in one month from last year. My computer had glitches early on after purchase last year and its motherboard was replaced, but this problem has remained, and the techs said it was a mail problem, not hardware.