A modern mail server should have features to prevent too many users from being present in the To: field. This will also prevent a lot of the Reply All problems we see regularly. This can be implemented as part of a Quota management system.
Problem is, options like these feel nice until they annoy you because you do in fact need more recipients. At that point you build a workaround into your software to send a new mail for every max_recipient users which brings you right back to square one with the additional technical dept caused by potential bugs in your batch sending code (off-by-one errors for example)
Just like one can have safe_parse() and unsafe_parse(), there could be an elevated API that could be safely used when vetted. For instance, when the SMTP server rejects a request it could put it in a special queue which requires manual confirmation.
This type of problems happen so regularly. In fact, a Japanese trader accidentally punched one too many zeros and lost a lot of money for the company. This type of problems can get very expensive in an instant. As an industry, we tend to think of input validation in passing because we don't pay enough attention to the problem domain.
Wow, I had to scroll a long long long way to finally get a reasonable suggestion on how to fix problems like this. A problem that potentially EVERY software has, regardless of framework or language. Kudos to you.
