AFAIK this isn't done already in an eclass for all mail-mta/* or ?
The reason why many mail-mta/* packages block each other is that they all provide the sendmail binary. So installing them at the same time would lead to file collisions. This does not apply to qpsmptd since it does not provide a sendmail binary. Why should it block the installation of other mail-mta/* packages?
oops - than this report is invalid - indeed.