'Excellency' it's the title/honorific, only to be used when you are referring or mentioning to the Aristocrat or if you are being presented to him/her, but if you are talking with him in a very formal/official context, then the correct usage (imo) would be: 'Excmo Sr/Dn (Don).' Most Excellent Sir/Lord or even as 'Don' (as a nobiliary particle plus his/her family surname or the name from where he/she get the nobliary title). Being this last title/honorific (Don) the most probably to be used in a more private, though not in a informal talk). E.g. '' His/Her Excellency the 'Viceroy' Lord X (and if he/she hold, too, a dukeship, then, at continuation it would be followed by) the 'Most Excellent Duke of X', will receive/meeting you at his private office'' and once there: the diplomatic would say/addressed to the ''Thank you for receive me so soon, Excmo Sr ('Señor', if not noble) and assuming that is a noble then Sir or Don. (insert noble surname)..'', followed by his/her nobility title or government office.Would you say "His excellency, the most excellent Duke of X" ?