What is wrong when I use xp_findnextmsg ?

I have Sybase 12.0 on Win NT 4.0 and using XP-Server I launch
successfully xp_startmail and I can send mails using xp_sendmail.

When I have unread messages and I used xp_findnextmsg none was found :

declare @out_msg_id binary(255)
select @out_msg_id=NULL
exec xp_findnextmsg @msg_id = @out_msg_id output

Server Message: Number 11526, Severity 20
Server 'XP Server', Line 0:
No message retrieved and/or deleted because none found.
(return status = 1)

We are using Microsoft Exchange Server and message type is X.400. The
messages comes on Machine where is the Sybase and I can read the messages
with Microsoft Outlook

When I am using xp_readmail without xp_findnextmsg I don?t
encounter error and the mail is mark from unread to read, but xp_readmail
variables remain to NULL status

Is something wrong with our message type ?