webmail (meaning a mail service accessed via a browser) doesn't even enter OP's spec
yes, I know. That was my main point. the OP didn't appear to have even identified exactly what they wanted to do or what requirements they needed to work within.

But defining either "suspicious" or "uncommon" with any particularity is going to be a major challenge
Agreed. The wiki link was intended to get the OP thinking about such possibilities, not as a conclusive list for making that decision. (I do admit my post was a little ambiguous on that point. )