user_id ZIP City    email
105 100051  Lond.   jsmith@hotmail.com
382 251574          jgjefferson@gmail.com
225 0100051 London  john.smith@hotmail.com

user_id ZIP City    email                   new_id
105 100051  Lond.   jsmith@hotmail.com      105
382 251574          jgjefferson@gmail.com   382
225 0100051 London  john.smith@hotmail.com  105

email<-c("jsmith@hotmail.com","jgjefferson@gmail.com","john.smith@hotmail.com")

dist<-stringdistmatrix(email,email,method="jw")
dist[dist==0]<-1

cbind(email,email_near=email[apply(dist, 1, which.min)],dist=apply(dist, 1, FUN=min))

     email                    email_near               dist
[1,] "jsmith@hotmail.com"     "john.smith@hotmail.com" "0.208754208754209"
[2,] "jgjefferson@gmail.com"  "jsmith@hotmail.com"     "0.281746031746032"
[3,] "john.smith@hotmail.com" "jsmith@hotmail.com"     "0.208754208754209"