Perhaps ask you provider what it is they think is spam... it would save a whole lot of guessing.

I presume this is caused by Mozilla because the test I ran only had a problem when it is sent from Mozilla. If I send it from Outlook (my email address is @hotmail.com) the problem does not occur.

I tried both a blank email and one stating "This is a test"

How do I know if this is being rejected by Outlook or Mozilla?

The message '550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)' originated from your chosen SMTP server. I assume that is the one for outlook, but I can not speak with certainty because I only know what you tell me about your setup.

The simplest of all is that the XX.XX.XX error codes are issued by Mail servers not mail clients. My guess and it is a guess is you have a signature in Thunderbird you do not have on the web site. They are notorious for causing false spam issues especially those with links to social media in them that Facebook F could be the cause or any of thousands of other possibilities.

Spam is a complex issue, made more complex bu a failure of definition. What Microsoft is flagging as spam is not the same as Google, is not the same as Yahoo and most certainly is not the same as what I consider spam. This is exacerbated by a trend originating in the USA to label all undesired mail as spam. The newsletter you subscribed to is not spam, but in the USA it is regarded as such because the customer says it is and they are always right.

I have the same problem. Works going out when I do a blind carbon copy bcc. Changed my signature to none. Anyone know how to contact MSN or Microsoft to possibly address an email server error or is this Thunderbird?

see my comments in this bug report from a couple of weeks ago https://bugzilla.mozilla.org/show_bug.cgi?id=1722916#c8