No, it really isn't a good solution, because not in all situations people will take their mails first before writing new e-mails.
Furthermore in such a case, most e-mail clients are not showing helpful error messages to the user - they often just state "SMTP connection refused" or something like that.
Ideally all mail clients would take mails first in the background always before sending a mail, but unless I get a mail client developer doing this, I think it is better to update the server supporting SMTP authentication ;-)
But confessed I don't know if this is just as easy as uploading a PKG or more complex and finally results in major OS modifications.
I fear that it is a complex issue however, otherwise someone surely had already presented a solution. Some time ago I directed this question to the pre-sales support of nuonce.net and earned only a reply mail "Not possible.".