I suspect you are referring to Act III, scene i, just after the Conspirators have murdered Caesar. Antony has made himself scare and sends his servant ahead of him to ask the Conspirators if he might speak with them. There is no letter, that I can see, involved. Also, when you say "he," I'm assuming that you mean the servant, but will also look at what Antony says upon his entrance into the scene at line 147.
Here is the upshot of the servants words:
Thus, Brutus, did my master bid me kneel,
Thus did Mark Antony bid me fall down,
And, being prostrate, thus he bade me say. . .
Say I love Brutus and I honor him;
Say I fear'd Caesar, honor'd him, and loved him.
If Brutus will vouchsafe that Antony
May safely come to him and be resolved
How Caesar hath deserved to lie in death,
Mark Antony shall not love Caesar dead
So well as Brutus living, but will follow
The fortunes and affairs of noble Brutus
Thorough the hazards of this untrod state
With all true faith. So says my master Antony.
So Antony, by way of his servant, says a few things. First he directs the servant to prostrate himself before Brutus, something one would usually do before royalty. In this way, he flatters Brutus as a "kingly" figure. The servant then describes how Antony loved Caesar, but will devote himself to Brutus and his cause if the Conspirators can satisfy him as to why Caesar "deserved to lie in death." In effect, he pledges to be a loyal follower of Brutus as he was a loyal follower of Caesar. The rest of the play and history lets us know that this is simply not true. It must be a ruse, a ploy, and part of Antony's scheme to overcome the Conspirators and gain power himself.
Upon his entrance after Brutus and Cassius agree to hear Antony, he has a speech in which he bids goodbye to Caesar and all but dares the Conspirators to kill him also if they wish, saying:
I do beseech ye, if you bear me hard,
Now, whilst your purpled hands do reek and smoke,
Fulfill your pleasure. Live a thousand years,
I shall not find myself so apt to die.
When they do not, Antony proceeds to "bloody" his own hands by shaking the bloody hand of each conspirator, all the while plotting his own ascension and revolt against Brutus and his co-conspirators.
For more on Antony and this scene, please follow the links below.