The letter from Azkaban arrives in a black envelope courtesy of a great horned owl.  It's large, and when opened, a great deal of paperwork falls out, and the actual letter from the prisoner is the least of its content.  With a flutter, a piece of paper will unfurl and speak its message in an intimidating voice.

Be advised, warned, and otherwise informed that this letter has been reviewed and censored for your protection.  Any return correspondence must have a return address and will be destroyed with out notification if evidence of any of the following are found: spells, enchantments, curses, hexes, jinxes, transmutations, transfigurations, runes, wards, blades of any kind, quills, pins, picks, files, or adhesives.  Escape planning is prohibited and could lead to criminal charges for both sender and recipient.

Good day.

Oct 5

Trevelyan - what the ████! I know you want to find it, but you have what you need.  I don't know what you're thinking but we had a deal.  Call off your dogs - don't send anyone else.  █████ ██ ██ if the ████ is █████ in within your authority, got it? 

Briggs

J. Trevelyan
Auror Office, Level Two
Ministry of Magic
London

Nathan Briggs
Azkaban

Mr Briggs,

Which dogs did you mean exactly?  As far as I know, I haven't sent any out recently.

You might want to use words with less or more than four letters in the reply, mate.  Looks like the censor's having a go at the rest of them.

Jonas Trevelyan
Auror

Again, in the response, the same bundle of warnings and provisions arrives with the small letter.

Oct 15

JOWD.