I found by a chance, that if you are using email encryption in GroupWise and try to open some encrypted email message in Retain you have problem. Because GroupWise is using only user based encryption and server based encryption is not possible, than archiving of such messages is not supported. Asked support if this will change in future and got answer, that there are no plans to change this.
This is only FYI, because this is logical and only because is not specified in documentation man can have impression, that it can magically work.
David