Welcome to the world of
radio. What you have is a proper
marine station license/call sign. It's the right number of characters, etc.
If you are trying to set up RMS Express to use Winlink, however, it is looking for an
amateur (ham) radio call sign, which is a whole different beast.
You can (let debate begin) use the same
SSB radio in
marine and ham
service, but you will need separate licenses and call signs for the two separate services.
Edit - For SSB e-mail your
marine call sign would generally be used with Sailmail or one of the other similar services.