Sylpheed is really a very good email client, but the passwords of the mailboxes are unfortunately stored unencrypted in a text file (accountrc, accountrc.bak, ...).
This is a big security risk!
Is there a setting that stores encrypted passwords? If not, I think it's better to use another email client!
You'd have to address that question to the publisher of Sylpheed. We just package it portably.
If you're carrying it on a flash drive, you could use a hardware encrypted drive like the PortableApps.com Carbide.
Sometimes, the impossible can become possible, if you're awesome!