Unable to send message
Alert Message An error occured while sending mail. The mail server responded 5.7.1 The message contains an unauthorized custom from address because the 5.7.1 associated account has been disabled or deleted. Please revalidate. 5.7.1 the custom from if you believe this to be in error. q2sm8373328wia.19 - gsmtp. Please check the message and try again.
Wšě wotmołwy (1)
So it looks like you're trying to send using a from: address which appears to have been removed, disabled or retired.
The message is from a server. You need to check if all the addresses offered in Thunderbird as from: addresses are current and valid.
But now I see other posts saying pretty much the same, which has me wondering if this is a problem at the server.
Who is your mail provider?
Wot Zenos změnjeny