I have 2 Gmail accounts in Mail. Yesterday I sent an e-mail from one of the recipients, after receiving the reply, the reply e-mail has got the sender as being from the other account. So it´s in the inbox of a different account I sent it from. I have noticed this wash´t the 1st time this happened; why ? How do I eliminate this from happening ?
The 1st time I noticed this I thought, maybe I didn´t switch accounts when writing the new message, but no, it´s some kind of bug.

[DCJ001]

I skimmed over your post. I have nothing to offer. Then, I saw this information about fixing problems with Gmail in Apple Mail:

Troubleshooting tips for Apple Mail on OS X Mavericks | Computerworld Blogs

I recommend taking a look to see if this helps.

OK, thanks. I´ll have a look.