Why do some of the Dark Lord's servants refer to him as "Sauron", such as his emissar, The Mouth of Sauron, and some of his agents sent to the dwarves:
"'As a small token only of your friendship Sauron asks this,' he said: 'that you should find this thief,' such was his word, 'and get from him, willing or no, a little ring, the least of rings, that once he stole. It is but a trifle that Sauron fancies, and an earnest of your good will [...]"
And I also seem to remember there is a chapter where some envoy calls him "Sauron the Great".
My problem with this is twofold: first, we know from The Silmarillion that Sauron's name wasn't chosen by him, but is instead a demeaning name given to him by his enemies:
The name Sauron (from an earlier form Thauron) originates from the adjective saura "foul, putrid" in Tolkien's invented language of Quenya, and can be translated as "the Abhorred" or "the Abomination". In Sindarin (another Elf-language created by Tolkien) he is called Gorthaur, "the Abhorred Dread" or "the Dread Abomination".
Instead, his original name is Mairon, "The Admirable". When he chose other names for himself, he chose pleasing ones such as Annatar, "Lord of Gifts".
Secondly, in The Two Towers Aragorn claims:
[Sauron does not] "use his right name, nor permit it to be spelt or spoken"
We see some confirmation of this when his Orcs refer to him obliquely, saying "the Eye" or "Lugbúrz" instead. Here there is some ambiguity about what his "right name" is, but in the context of Aragorn's assertion it seems to be "Sauron" (since they are discussing an "S" rune painted on some Orcs, and they decide it must refer to Saruman instead).
So why would he give permission to some of his subjects to use his name, contradicting Aragorn, and -- when faking friendship -- why would he choose a name his enemies would understand to be insulting?