Gittin, Chapter 1, Mishnah 3. In the first Mishnah we learned that the requirement to declare "b'fanai nichtav u'v'fanai nechtam" ("it was written in my presence and signed in my presence") applies to one who brings a get from overseas to Eretz Yisrael (and perhaps even in the opposite direction). Our Mishnah completes the picture and deals with two cases: one who brings a get within Eretz Yisrael, and one who brings a get from overseas but is unable to say "b'fanai nichtav u'v'fanai nechtam".
One who brings a get within Eretz Yisrael:
"HaMeivi get b'Eretz Yisrael eino tzarich sheyomar b'fanai nichtav u'v'fanai nechtam" - an agent who brings a get within Eretz Yisrael is exempt from this declaration, and is not required to testify that the get was written and signed in his presence.
"Im yesh alav orerim yiskayem b'chosmav" - when a question arises about the validity of the get, and specifically when the husband comes and claims that the get is forged, its authenticity must be established through those who signed it. This is done in one of two ways:
The signing witnesses themselves confirm that these are their signatures.
Other witnesses who recognize the signatures confirm that these are indeed the signatures and that there is no forgery here.
But as long as the husband has raised no challenge against a get brought within Eretz Yisrael, the woman may rely on it: there is no need for the declaration "b'fanai nichtav u'v'fanai nechtam", and there is no need for witnesses to authenticate the signatures - the signatures retain their presumed validity.
One who brings a get from overseas and is unable to make the declaration:
"HaMeivi get mimedinas hayam v'eino yachol lomar b'fanai nichtav u'v'fanai nechtam" - one who brings a get from overseas, the case discussed in the first Mishnah, is required to say "b'fanai nichtav u'v'fanai nechtam", but here we are dealing with someone who is incapable of saying it. When does this occur?
When he became mute or deaf.
And some explain: even when he did not actually witness the writing and the signing.
"Im yesh alav eidim yiskayem b'chosmav" - in such a case, if there are witnesses who can authenticate the signatures, the get is validated through them.
The Gemara's difficulty according to Rava:
The Gemara examines how this Mishnah fits with the view of Rava, who explained that the problem with a get coming from overseas lies in the fact that outside Eretz Yisrael there is insufficient awareness of the law of "lishmah" - that the get must be written for the sake of the parties, for the sake of the husband and the wife. Seemingly, authenticating the signatures does not address the problem of "lishmah".
The Gemara answers that our Mishnah deals with a later period, "l'achar shelamdu" - after the people outside Eretz Yisrael had learned the law of "lishmah", so that there is no longer any concern in this matter.
Even so, l'chatchilah one must still say "b'fanai nichtav u'v'fanai nechtam", because the Sages did not annul their enactment, out of concern that matters might revert to their earlier state and people would once again be unaware of the law of "lishmah". They therefore kept the declaration in place even after the law had been learned. However, in an exceptional case, where the person is already unable to say it - such as when he became mute - the Sages did not insist on their enactment, and it is sufficient to validate the get through those who signed it.
In summary: in this Mishnah we learned that one who brings a get within Eretz Yisrael is exempt from the declaration "b'fanai nichtav u'v'fanai nechtam", and only if there are challengers - primarily a claim of forgery on the part of the husband - must the get be validated through those who signed it, whether by the signing witnesses themselves or by those who recognize their signatures. Regarding one who brings a get from overseas and cannot make the declaration, the get is validated through witnesses, and according to Rava this refers to the period after the law of "lishmah" had been learned, in which the enactment remained in force l'chatchilah out of concern that matters might revert to their earlier state, while in the exceptional case the Sages did not insist upon it.