• Open Mail.app
• Mail > Add account
• Select “Other Mail Account…” and click “Continue”
• Enter your Email Address and SDF password.
• Next menu displays “Unable to verify account name of password.” Fear not! Enter Username: <your SDF username> Password: <your SDF password>, Account Type: IMAP, Incoming Mail Server: <mx.sdf.org>, Outgoing Mail Server: <mx.sdf.org>
• Click Sign In. Check box for Mail. Uncheck box for Notes. Click Done.

OK, now you can receive mail but not send.

• Try sending yourself an email. It won't send; it will be in the Outbox.
• Click on that message in the Outbox. It displays a menu entitled “Cannot send message using the server SDF.”
• Select SDF and click “Edit SMTP Server List”.
• Enter Username: yourusername.sdf.org@sdf.org, Password: <your SMTP auth password>, uncheck “Automatically manage connection settings”. Port: 587, check “Use TLS/SSL”, Authentication: select “MD5 Challenge-Response”.
• Close the window. Select SDF server and “Try with Selected Server”.