إِلَّا مَنِ ٱرْتَضَىٰ مِن رَّسُولٍ فَإِنَّهُۥ يَسْلُكُ مِنۢ بَيْنِ يَدَيْهِ وَمِنْ خَلْفِهِۦ رَصَدًا
Illaa manir tadaa mir rasoolin fa innahoo yasluku mim baini yadihi wa min khalfihee rasadaa
except to a messenger he has chosen — and then he sends watchers before him and behind him
This closes a loop the surah opened: the jinn began by reporting that the stations they once used to eavesdrop on heaven are now guarded, and the guard reappears here around the message itself. Rasad names a watch posted to intercept. The exception is narrow, one chosen messenger, and even that channel is escorted at both ends.