Gmail is easy. we have automated setup for Gmail, Yahoo, and outlook.com, but any email will work. But, yes, no linux client yet. Sorry. You can use on your Android phone, Mac, or Windows PC, though. :)

Took us over two years, but we did it right. https://www.securemyemail.com.... Works with your current email address(es). Simple enough for anyone and everyone but with advanced PGP key management features for those that like that sort of thing. You can even import your current key as well as invite legacy PGP buddies w/o them having to use. Easy inviting and optional updated web of trust using social networks is built-in. Android and Thunderbird are available now. iOS, our own simple clients for Mac and Windows should be out, we hope, by March and April.Apple Mail plugin and Outlook will follow. 30 day free trial and only 99 cents a year per email account for unlimited use. all systems are in Switzerland and it's truly end-to-end encryption where even we don't have the means to decrypt. ever. and for those in-th-know, no webmail so no javascript vulnerabilities or other shenanigans are possible.