We have a database app that emailed a daily summary, as a big one-line blob of HTML.
We saw the spurious '!' insertions in Outlook. They were not being sent by the app.
I clicked on Outlook’s “If there are problems with how this message is displayed, click here to view it in a web browser”, then in the browser right-clicked “View Source”, and I found that
two spurious “!\n ” had been inserted:
<th
style="text-align:rig!
ht;"><a>Sites</a></th><th
style="text-align:left;"><a>Last No!
te</a>
This spurious "!\n " insertion happened around charactet 2039 of the single long line of HTML.
The spurious insertion does not occur if I run the HTML through a formatter which indents and inserts line breaks between HTML elements.
I hypothesize that the Outlook preview browser has a buffering problem in its parser.