You could use the "Keywords:" header (sometimes used in Usenet) if you want something like tags. For suggested short responses, perhaps make up a new header su

You could use the "Keywords:" header (sometimes used in Usenet) if you want something like tags. For suggested short responses, perhaps make up a new header such as "Responses:" with a comma-separated list of short responses. This would make it fully compatible and can work fine with plain text and will be compatible with all existing email clients and servers. It is also not that complicated to implement, and some users may see the headers already.