I had the same problem. My emails were being returned. This was after years of using avast and thunderbird and usinhg my current internet provider Vidahost. i thought it was because Vidahosts servers were blacklisted. Vidahost looked into it and came back with they thought it could be my anti virus. I switched Avast off and everything worked fine. Since then I have been into Avast and switched it back on but switched off the outgoing mail scanning (settings|components|mailshield|customise| and unticked scan outbound mail.) Everything worked. The reply that I got from my ISP explained it like this.
"Avast changes HELO to a value, that is a default one and is usually used by spammers, so whenever our system detects it, it routes the messages through specific IPs, which, due to the reasons above, often gets blacklisted." Not saying your problem is the same but maybe. Why it should suddenly start is an interestiing question. Maybe a change in thunderbird or avast started it.