In the previous mishnah we discussed a case of someone who brought a get within Eretz Yisrael and fell ill on the way: he may send the get with another messenger, because we assume the husband would agree to the appointment of an additional messenger, since he himself would have appointed someone else in his place in such a situation. Why was this ruling stated specifically regarding the delivery of a get within Eretz Yisrael? Our mishnah explains.
The text of the Mishnah:
"HaMeivi get mimedinas hayam vechalah - oseh beis din umeshalcho, ve'omer lifneihem: befanai nichtav uvefanai nechtam. Ve'ein hashaliach ha'acharon tzarich sheyomar befanai nichtav uvefanai nechtam, ela omer: sheliach beis din ani" - one who brings a get from overseas and fell ill convenes a court and sends the get with a messenger, and he states before them: it was written in my presence and signed in my presence. And the last messenger need not say it was written in my presence and signed in my presence; rather he says: I am a messenger of the court.
The special problem with a get coming from overseas:
The unique situation here is that one who brings a get from overseas is required to say "befanai nichtav uvefanai nechtam" - a declaration he is not required to make when delivering a get within Eretz Yisrael. Hence the problem: he cannot simply appoint another messenger in his place as in the previous mishnah, since that other messenger cannot say "it was written in my presence and signed in my presence," for he was not present at the writing and the signing.
The solution - appointment through a court:
"Oseh beis din umeshalcho" - the messenger who fell ill convenes a court, or comes and appears before one, and the court is the body that appoints the additional messenger.
"Ve'omer lifneihem: befanai nichtav uvefanai nechtam" - the first messenger makes his declaration before the court, and in this way the testimony is heard as required.
"Ela omer: sheliach beis din ani" - the last messenger need not say "it was written in my presence and signed in my presence"; it is enough for him to declare that he is a messenger of the court.
The reason for this: once the court is the one making the appointment, we assume it did its job properly - that it received the get and heard the testimony of "it was written in my presence and signed in my presence." Therefore, when the last messenger represents the court, we know that the matter was handled correctly and that the declaration was indeed made.
The Gemara's inference from the phrase "the last messenger":
The Gemara notes that the Mishnah did not say "the second messenger" but rather "hashaliach ha'acharon" - the last messenger. From this we learn that the second messenger too, if he fell ill or was prevented by circumstances beyond his control, may appoint a third messenger, and the third a fourth - even up to a hundred messengers. However, each and every time this must be done through a court, since the last of them must be able to say that he comes with the authority of the court.
In summary: all of this applies specifically to a get coming from overseas, where the declaration "it was written in my presence and signed in my presence" is required, and therefore the appointment of a replacement messenger requires a court. Within Eretz Yisrael, by contrast, where this declaration is not needed, a messenger may appoint an additional messenger on his own, as we saw in the previous mishnah.