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You should be able to use the following setting for your smtp-server in ~/.pinerc:

smtp-server=smtp.mac.com/user=YOURUSERNAME

(Replace YOURUSERNAME with your .Mac user name.)

If you get an error message about an invalid certificate, you may want to try appending novalidate-cert, which tells Pine to accept smtp.mac.com's SSL certificate, even though it cannot be verified by a certifying authority:

smtp-server=smtp.mac.com/user=YOURUSERNAME/novalidate-cert