System.Net.Mail does not support implicit SSL which is required connect via SMTP via most ISPs since last year (e.g., Verizon).
This is actually NOT a shortcoming of Mono as Microsoft does not support it either but since due to its prevalence they may need to. I would hope that Mono may take the lead and inspire them.
The only difference between explicit and implicit SSL is that with implicit, SSL must used to established the initial socket connection as connection to a normal socket and then switching.
my last sentence should read...
The only difference between explicit and implicit SSL is that with implicit,
SSL must used to established the initial socket connection as opposed to with explicit where a normal socket is established first and then the conversation is immediately switched to a secured one.