The SITA priority codes are essentially defunct these days due to better, wider and more resilient networks. You do not necessarily pay more for higher priorities either these days (QD QU QX are the same price now) either. Email is fine for some messages traditionally sent via the Type-B network, but you do lose the data security aspects; anyone can edit the content of an email. Just imagine a flight plan sent via email and some malicious ops guy with a motive editing it!!