Sent e-mail not saved in IMAP "Sent" folder

With 2 different e-mailers (Entourage and Eudora), my sent e-mail is not saved to the IMAP “Sent” folder. I have the proper folder selected in the settings of the e-mailers. When I send a message, both e-mailers attempt to file the sent message into the sent folder, but always return an error message. Entourage has an “Error 1025”, with the following text:

"The server returned the following error:

Unable to add message to IMAP mailbox. The corresponding local folder will be used instead."

Can you tell me what I’ve set up incorrectly that is preventing my e-mail from being filed in the Sent folder? I’ve already assured the file names are correct (I browsed to choose the correct folder).

Thank you.