you currently use the server type "standard unix mbox spool file" and have

entered /var/spool/mail. /var/spool/mail is a directory and not a file. you

could also use "standard unix mbox spool directory" to point to a directory.





"Could not open mail file /var/spool/mail: Is a directory"





When you attempt to choose your mmox file in /var/spool/mail, hit <ctrl>L to bring up the location text-entry box 

(if it wasn't already visible). 

Then type in the name of the mbox file you want (typically your username).

THEN add a forward-slash then hit return. i.e. type in "username/". Then the job is done!



Unfortunately, you need to repeat this EVERY time you open the configurations dialog for that mail account.



This is an evolution bug in the type of GTK dialog box they've used. 

The dialog box is set to only select directories but mbox files are (obviously) files. 

Putting a slash on the end kids GTK into accepting this as a directory.