Proposal: Check mailings for fixed personalization (without tokens) before submitting
Sometimes, people copy a link that was personalized to them with cid and cs and insert it into a mailing (i.e. with their contact ID and checksum instead of the appropriate tokens), then they send out a personalized link for their contact to a large group of people.
In order to prevent this, we could check for URLs that include something in the form of a checksum plus cid=integer or cid1=integer (this will support Webform CiviCRM too). Optionally, we could even validate the checksum, but that seems overkill. I think this would make the most sense at submission time, to keep things simple with Mosaico and Traditional mailings.
I'm not sure what the best way to handle the UI for this is. An error that prevents sending seems too strong, but just a warning while still submitting seems too weak. Is there a better UI solution for this?