I have downloaded and installed a new font family (Gentium) on my machine. I want to use this font in some of my email correspondences. When I compose an email in Outlook 2007 with these fonts and send it to someone, is he going to be able to display it properly (I mean, with the same fonts, just the same as I can see the email)? I'm assuming the addressee doesn't have this font family installed on his machine. Does it matter if the addressee uses Outlook or a web-based email?
Short answer: No.
A font installed on your computer will unfortunately not be available on other computers (*)
But your text will in any case still be readable. Only downgraded with another substitute font available on the remote computer.
It doesn't matter which email client you use (web-based or not)
You may use some CSS3 @font-face styles to do it. But this is way beyond the standard usage for an email. And, at the time this answer was written, it is mostly unimplemented or buggy in email clients out there.
(*) Unless it has that font installed too, which is very unlikely.
If the recipient doesn't have the same font installed on their machine as the one you've composed with in your email, then they won't be able to see it as you do.
Their program might strip it down to plain text or it might just approximate the font in the email for the closest possible match on their end.
Still, they might have another font hard set to display all their incoming emails, which again renders your font choice a slim chance of viewership.
Worse case is where the entirety of the email is just glyphs because it's not being rendered correctly on account of the missing font file.