Using Evolution 1.4.5 with a 'Standard Unix mbox spool or directory'. Mail is not being removed from the file. I do not see an option to uncheck/check, as mentioned in the post 'POP mail not being removed'.
that's because you are yusing the wrong account type if you ant it to be removed from the spool. What you want is 'Local Delivery'.
If you read the descriptions of the 2 in the account druid (or editor), you'll see that they explain the differences.
I was using 1.4.4 and never had this problem. 1.4.5 is a recent 'upgrade'. I cannot get back to the older version.
1.4.4 didn't remove the mail either. it's worked the way it does in 1.4.5 since 1.0
Jeff
