Today No-IP lauched SSL encryption for Alternate-port SMTP. Now clients like Outlook and Mail.app can use an encrypted channel to submit their email. Simply set the correct port and turn on SSL encryption in your client and you are done! The ports are these:

Outlook and Outlook Express (implicit SSL clients)
Use port 465 and check the Use SSL Encryption check box.

All other modern email clients, STARTTLS enabled (explicit SSL clients, e.g. Thunderbird, Mail.app, Evolution)
Use port 3325 or port 587 and check the Use SSL Encryption check box.

This information can be found in the No-IP FAQ item, Does No-IP provide encrypted mail transfer (SSL)?

And, as a bonus, here is a little geek knowledge. Implicit SSL means that SSL is implied, that from the moment the connection is made all data is going to be encrypted. Explicit SSL means that the client must explicitly request SSL. Basically, when a connection is made the server announces that it can do SSL and the client says that it wants it. From that point forward the channel is encrypted.

Implicit SSL is used for SMTPS on port 465, but that standard is deprecated in favor of the more adaptable STARTTLS (RFC 3207). TLS stands for Transport Layer Security and has become a standard for protocols like SMTP. STARTTLS offers the convenience of one port with both plain text and encrypted channel and in that right is more versatile than implicit SSL. Most modern email clients support STARTTLS, save Outlook and Outlook Express. Perhaps in the next version.