Apparently I cannot receive or send emails from the company that handles our complaints. I get the same error message when sending.
An error occurred while sending mail. The mail server responded: Requested action not taken: mailbox unavailable invalid DNS MX or A/AAAA resource record. Please check the message recipient.......and try again.
Wšě wotmołwy (1)
This error is about DNS records and is from the providers server. Contact them since this has nothing to do with Thunderbird other than it delivered the message.