Have you contacted you mail providers support. You have contacted support for Norton and your Mail client (Thunderbird), but I see nothing to indicate you have contacted the ones rejecting your mail.

I would suggest that this has nothing to do with spam or a virus. It probably has to do with a poorly configured SMTP server that you actually only get when the load on your providers servers get to the point you get passed to it.

Why does it think you have spam content in your mail. I have no idea. I have seen some very brain dead reasons providers have configured just such a response. From including a reply-to header to you have simply exceeded their daily allocation of mails. To know you need to ask them.

Matt - thanks for the reply. I use GoDaddy to host email and did contact them. They said it was with Thunderbird, as sending email using their Webserver email worked fine. I believe I have a worm or virus due to all of the bogus email delivery notices that I am getting, which is getting worse; 8 last night, all at 2:40 am. I have Norton 360 on everything I use except the IPhone. The only device I have turned on at that time is an IPhone, and I do access this email account with that. Appreciate any guidance you can offer. I have attached one of the Mail delivery failed messages.

I think there are issues within issues here.

First, The returned messages.

Unless you are operating a domain, or have an email address set up with smartape.ru then the mail you are receiving the non deliveries for is not originating with you. It starts it's life sfom what you posted at smartape.ru.

I would suggest this is most harmless and is best described by Wikipedia in their article on backscatter. See https://en.wikipedia.org/wiki/Backscatter_(email)

Second to Norton. Basically they are useless at email scanning. They do not scan IMAP mail accounts, not do they scan mail on encrypted connections (SSL and TLS). In this they still suffer from all the old issues caused by anti virus products scanning email. So you get all the bad and none of the good. Just remember they are not scanning your mail and you will be alright.

Third the real issue. That I think is the stupid manner in which godaddy are setting up their servers. SO we will see if this fixes the issue with them

Open the config editor and type in mail.smtpserver. In the list of preferences below will be probably more than one entry for mail.smtpserver.smtp[^].description where the ^ is a number. Identify the correct number for the account using the description to identify which account is which. Also note the value of the preference hostname for that server. This will probably be a server name such as SMTP.secureserver.net

Now in the list of items in the config editor. Right click and select New and then String from the menu. Add the preference mail.smtpserver.smtp{^].hello_argument and the value from the hostname preference.

Note the preference will look like mail.smtpserver.smtp9.hello_argument not mail.smtpserver.smtp[9].hello_argument

I really do not know if this will fix the issue, but have a suspicion that it will. Please let me know