Thunderbird is just an email client. It does not check for mailbox full, and does not have any block limit, nor does it verify sender. All of that is done by your email provider and, since Thunderbird is your agent, Thunderbird dutifully delivers those messages to you. I suggest you check with your email provider on these errors. The serder verify error could possibly be within thunderbird if the account is not set up properly. On that, you might check the email address of the account and the email address of the SMTP server settings pane to see if they match and if the full email address is there.