Why are long deleted emails reappearing?

[Android Central Question]

I'm using Thunderbird on my PC and it's sister app Blue Mail on my Galaxy S7 phone. Every few weeks, emails that I've already read and deleted on either PC or phone, suddenly show up in my Thunderbird inbox again as "unread," sometimes as many as 2000 of them.
Is this an Android problem, a Samsung Problem, or a Mozilla problem?

 

[methodman89]

I'd look at the server sending them. Unless you have a backup setting on, and they're just doing as told. I'd email the email app developer and ask about compatability and sync settings.