If you want a mail library that is guaranteed to work on Mono, I suggest using
Most of its development is done on Mono and it supports S/MIME and OpenPGP even on Linux (something no other .NET mail library does).
MailSystem.NET, unfortunately, is very broken WRT rfc compliance. I've pointed out numerous problems in the forums and the problems haven't been fixed in the last 6 months since I ranted about them (to be fair, it seems like the project is mostly dead at this
point, so that probably explains why).
MailKit's MIME parser (aka MimeKit) is almost 2 orders of magnitude faster than MailSystem.NET's parser and is a proper parser to boot - it's not a string.Split()/Regex/Substring monstrosity, but rather a proper tokenizing parser so you can be sure it is a
lot more rfc compliant. The same goes for the IMAP implementation in MailKit.
It's just better in every possible way.
I wouldn't normally post this to another project's forum, but because MailSystem.NET seems like its a dead project, I figure I'm not likely to offend anyone.
Hope that helps and hopefully no one is offended.