Problems Sending mail via GMS

Hi,

we have a Groupwise Mobile Server, working nicely.

However with one phone, the Sony Ericsson K800i, we have the following
problems:

The user can receive mails and reply to them. In this case the mail is
delivered and all is ok.

However, when creating a new mail and sending it, it looks like it is sent
but never arrives at the recipient.

Any idea what this could be?

Thanks a lot.