You don't need an add-on for that. I have a plain TB 91 setup with three identies configured for three different mail servers. When replying I can choose from the identities, and in the drop down there is another option "Customize From Address" which enables editing the address without creating another identity. It's a bit hidden, but should be there without an add-on. Doesn't work for new mails though, in this case the option is there but disabled.
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.)
