While Fulton recognizes that "Paul had a preference for including others as co-senders" (p229), she shows that the phenomenon was much more common than many suppose. She finds that about 20% of ancient letters had co-senders. Official letters, in particular, frequently had co-senders (p152).
The role of co-senders
Fulton corrects a lot of misinformation about the purpose of including co-senders. They were neither letter-carriers (p212-4), nor secretaries (p214-6). They were not generally co-authors either (p216-9). So what role did they play? Fulton concludes: "Rather, it would seem that those named in the letter prescript as the senders of the letter are those who take responsibility for the contents of the letter." (p218).
She finds that Paul's co-senders were no exceptions:
"His co-senders joined him in taking responsibility for the letter and would be perceived by the recipients as being in agreement with its contents." (p229)
It is therefore not surprising that, when Paul writes to a church, he includes as co-senders those who had helped him establish that church. Timothy and Silvanus, naturally had authority in the communities that they helped to found, so their endorsement of Paul's letters carried weight. Thus Fulton writes:
"In selecting co-senders Paul generally selected someone who not only worked alongside him but was also known to the recipients and involved in founding the church."(p181)
Similarly she writes:
"From the viewpoint of the recipients of the letters, the co-senders, like Paul, were part of the team who founded the community." (p174)
But then she says, without evidence, that "Sosthenes may be in a different category" (p174). This seems to me to be special pleading. Acts 18:8 reads:
"Crispus, the official of the synagogue, became a believer in the Lord, together with all his household, and many of the Corinthians who heard become believers and were baptized."
It seems to me that Crispus-Sosthenes was a co-founder of the Corinthian church, in that many Corinthians followed his lead. His founding role also explains why he was given the name "Sosthenes" (saving strength), and why he was beaten (Acts 18:17), and why Paul chose him as his co-sender. Crispus was just the kind of person whose endorsement Paul would want for his letter to Corinth. Fulton's observations about the role of co-senders therefore supports the view that Sosthenes was Crispus renamed.
Timothy is Paul's co-sender in every letter except Galatians, Romans, and 1 Corinthians. These exceptions are explicable since Timothy did not help to found the churches of either Galatia or Rome, and he was already on his way to Corinth when Paul wrote 1 Corinthians. It is significant that, in the other letters, Paul consistently seeks Timothy's endorsement. This, along with texts such as 1 Thess 3:2 and 2 Cor 1:19, demonstrates that Timothy had more authority than the Pastoral Epistles suggest (e.g 1 Tim 4:12).
It seems to me that Paul's choice of co-senders confirms that churches generally respected their founders because they were their founders. This, in turn, supports my hypothesis that the agitators in Galatia were appealing to Paul's (not Jerusalem's) authority, saying that Paul had yielded to their view that circumcision was necessary.
The use of first person singular and plural
Fulton finds that epistolary plurals are rare. She also observes that Paul uses the singular (I, me) more often than other letters that have co-senders.
The closing of a letter, where the author (or one of the authors) writes in his own hand is called the "subscription" or "autograph". Fulton writes,
"In all of the extant Pauline letters the autographs are generally attributed to Paul alone, even when the letter is from Paul and others. ... this deduction seems reasonable"(p202)
This raises the question of whether Paul's co-senders would have been held responsible for the contents of the autographs. Did the readers assume that the co-senders endorsed the body of the letter, but that the author alone was responsible for the autograph? If so, Paul may have reserved his severest rebukes for his autographs in 2 Corinthians and Philippians so as not to jeopardize the relationship between Timothy and the churches. See here.
The role of letter carriers
Fulton suggests that letter carriers could pass on information that could not be safely put in writing:
"more 'sensitive' matters were dealt with by verbal messages rather than by written messages which could be intercepted." (p130)
Name order (p159-60)
In the new testament people are invariably listed in descending order of their importance (to the context). Likewise, Fulton concludes that, from the letters that she has examined, co-senders are generally named in hierarchical order. She finds 7 letters that name a female as well as a male co-sender, in only one of these cases is the female mentioned first.