Thunderbird
I use Mozilla Thunderbird as my desktop email client on both GNU/Linux and Windows 10.
As of Thunderbird 78, there is built-in support for both OpenPGP and S/MIME.
I use the following keywords to warn me when I might have forgotten an attachment or link: attach, attached, attaching, attachment, cover letter, CryptPad, document, Drive, Google, letter, LibreOffice, link, ODT, PDF, PowerPoint, send, sending, slides