I&#x27;m pretty sure Postbox would tick all of those boxes.  I definitely use non-GMail accounts and multiple signatures - each account has a different default signature, and I can change the signature on individual messages on the fly.<p>Not sure about encryption, but it&#x27;s bound to have a PGP plugin or something similar.  Postbox is partly derived from Thunderbird, so it would probably satisfy your requirements as well.<p>Edit: Just realized that I&#x27;ve assumed Apple Mail refers to the Mac version.  I&#x27;m not sure about iOS, but the new Microsoft Outlook (rebranded Acompli) might do some of those things.  Personally I use mobile only for triage, and write emails when I&#x27;m on my laptop.