Error "Could not connect to port"

I created a virtual printer, using the driver for my USB-connected printer, to forward print jobs by email.
When I print to that virtual printer I receive this error message. However, the print job is still sent to the email address I specified.
What does this error message mean?

Second question: print jobs will be sent as files with .prn extension. I wouldn't know how to handle or process these. Can I choose another file format?

Nico,

Were do you see this error message, is it in the Print Distributor log, if so what does the rest of the message say.

If you are using a PostScript compatible printer then you can use Adobe Acrobat or GhostScript to convert it to a PDF file before you email it.