SMTP is fine, as long as you can safely assume your users will all have access to it.
Even when it's available, it may not be the user's choice.
I, for one (and I believe a lot of users feel the same way) would prefer to use my web-based email account.
If you can add that as a configuration option, that would be ideal.
You could probably leverage modules such as Mail::Client::Yahoo and WWW::Hotmail to handle the nasty bits for you.
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