Which email program works best with templating?

Question

Mail client

I don't want to lose my old box by switching to a new one.
I really like to send beautiful letters in the right template.
Ideal option: a mailer that supports any number of accounts and any number of templates in any account.
What I checked:
Thunderbird - added a templating module, it’s not bad if the mailer itself didn’t slow down so terribly for some reason, one for one
Google Mail account - there is templating, but only one for one account - set up all letters go in the same
Microsoft Outlook 2007 template - it takes a long time to load, it works slowly, for some reason the built-in non-switchable scheduler. The template function seems to be somewhere, but it was not possible to find it in this sracha from Microsoft.
Maybe there is a solution?