re:I'll get an email from Google about a security alert (when logging in through my browser),

that is normal

re:Thunderbird will send a reply to 3hxUVYwgTDSYPQ-TGRNaCEEQWPVU.IQQING.EQOTaCP.T.IWaVQPIOCKN.EQO@gaia.bounces.google.com.

If that is received into your 'Inbox' then google sent it to you.

It might be telling you that an email was sent using your email address and it was bounced for some reason eg: no such email address If you are not sending those emails then maybe someone is abusing your email address. As far as google knows in the email the 'FROM' says its from you so google bounced it back to you. But it might not be you who actually sent it.

This is not normal. These emails aren't being sent TO me FROM Google, they are being sent FROM me TO Google, but appear in my inbox, not my sent folder. Looking at my gmail account, these emails aren't there, neither in my inbox or sent folder. These emails only appear in Thunderbird.

So I am indeed the idiot.. Somehow my "Vacation Responder" option was turned on in my mobile Gmail app, and didn't have an end date or message to go with it, so that's why these messages were being sent and would bounce back. I didn't see them in Gmail because Thunderbird is set up through POP and would delete them off the server before I could see them.

So I am indeed the idiot.. Somehow my "Vacation Responder" option was turned on in my mobile Gmail app, and didn't have an end date or message to go with it, so that's why these messages were being sent and would bounce back. I didn't see them in Gmail because Thunderbird is set up through POP and would delete them off the server before I could see them.

Many thanks for the feedback, it might prove to be very useful to others. Could you mark your comment which explains your solution as the 'Chosen Solution'.