Att is an abbreviation of the English word "attention", which in Portuguese means "attention".
Corresponds to the abbreviation "A/C" (be careful) that is written on documents or orders whose recipient is to be specified.
It is very common in formal emails o use of the final greeting "Sincerely" also reduced to the abbreviation "Att".
This expression is usually written before the signature, as a way to end the message in a delicate and polite way.
However, the short form of the expression "Sincerely" is, by convention, accepted as "At.te" and not "Att", as it is usually written.
Some style guides, however, recommend that the most delicate and elegant way to finish a message is to write the term "Sincerely" in its full form, without using the word abbreviated.
It should be noted that the form "attention" (be careful) used in English-speaking countries, to specify the recipient of the message is correctly abbreviated as "Attn:" (without forgetting the colon).
In some cases, the abbreviations "FTAO" (for The Attention Of) or simply "FAO" (for attention of).
In English, att can also be the abbreviation for attached ("attached") or attorney (" attorney").