As I mentioned, your message shows that your ISP's SMTP server has acknowledged receipt of the complete delivery of the message to the server. The failure to then deliver is for the ISP to explain.
However, I recognize that you are dealing with support folks at the ISP who are not well trained in understanding how SMTP really works or how the message delivery interacts with an antivirus scanner such as avast.
As you have seen, my ISP delivers these messages, so should yours but - for whatever reason your ISP's mail system does not (and it appears that at least one other ISP does not - from this thread).
Rather than beating your head against your ISP it would seem the course of least pain would suggest:
1) Either turn off the outbound scanning of emails in the avast Internet Mail provider
or
2) Select different settings in Thunderbird that will still send an empty message body (as seen by your recipients) but not -hopefully- provoke the same problem in your ISP.
Is there a specific reason why you wish to send a plain text empty messsage body?
If you select the option for your mail account to compose in plain text and HTML then Thunderbird would provide both the plain text and html equivalents of an empty message body in the alternative format. Your recipient will see an empty message body no matter whether their client suppports html or plain text rendering.
