problem archiving

Dear all, 

I try to save outgoing messages in a unix mailbox file ~/mail/sent-mail. I 
have created a nndoc group called nndoc+/Users/harven/mail/sent-mail:sent-mail
Here are my settings in my .gnus.el file:

(setq gnus-message-archive-method '(nnfolder "archive"))
(setq gnus-message-archive-group "nndoc+/Users/harven/mail/sent-mail:sent-mail")

When I send a message, I get the Gcc header
Gcc: nndoc+/Users/yves/mail/sent-mail:sent-mail
but I get the following in the message line:
Couldn't store article in group nndoc+/Users/yves/mail/sent-mail:sent-mail: 

The file sent-mail has all permission set. Yet, the message is not
stored in the mailbox. What I am doing wrong here ?