If that doesn't work, then try this...
Get an email that WORKED, then do a "View Source" and copy and paste the html code in that email into your B4A code and set that code to the ".Body" and try sending the email again. So, in effect we are just simply re-sending known good HTML code.
If it then works, then you know it is something in the html source that is the key to getting this working and you'll just have to reduce it more and more to narrow down to what the "fix" is.
If it doesn't work, then compare the "View Source" of the bad email to the good email and see what the difference is.