Restrict number of external recipients in to/cc fields - Gsuite?

[ADMaster]

I know this is trying to find a technical solution to a people issue but…

I’ve tried searching but just get gmail sending limits help articles.

Also I’m in the US so thankfully not a GDPR issue.

 

I found out today a teacher has emailed all the parents with their address visible to the other parents. I send out a reminder to use BCC a least once a year. I’ll have to do so again next week.

 

I’m thinking I could write a regex if x number of external recipients are in the to or cc fields then block. Customize the block message telling them to use BCC.

 

If anyone has already done this please share the regex or other solution.

 

Its late on a Friday, so I’ll put more thought into it next week.

 

Thanks