Comment 1 for bug 1573666

I am less certain about presenting anything to the user in the catalog.

As for other situations, if you know an email domain has restrictions (vtext.com converts to SMS, for example) putting a warning up would make sense. "This email address will only receive the first 120 or so characters in the message" (losing some to subject and such in this case).

If this were implemented as "send the potential address to the server, see what the server sends back" then (optionally) having the server check if there is even an A/MX record for the domain portion may be a good idea as well.