GP (and I) don't want to configure multiple 'identities' though. I (and probably GP) create new email addresses 'on the fly', for a specific shop I order from once say, and if there's something I need to reply to I want (possibly need from their CRM's perspective) to reply from the address they already have for me.

Embarrassingly largely for this reason, I wrote my own client. (Good fun though, and not the only reason.)