Mail <- c("xxx1@xxx.xx", "xxx2@xxx.xx", "xxx3@yyy.xx", "xxx4@zzz.xx")
InterestingPublishers <- c("zzz.xx", "xxx.xx")

Mail  %in% InterestingPublishers
FALSE FALSE FALSE FALSE

grepl(InterestingPublishers, Mail)
Warning message:
In grepl(InterestingPublishers, Mail) :
  argument 'pattern' has length > 1 and only the first element will be used

containsi <- integer()
for (i in InterestingPublishers) {
  containsi <- c(containsi, grep(i, Mail))
}
Mail[containsi]

[1] "xxx4@zzz.xx" "xxx1@xxx.xx" "xxx2@xxx.xx"

Mail[unlist(lapply(InterestingPublishers, function(x) grep(x, Mail)))]