Mail delivery failed: returning message to sender

Hello everyone, are familiar the error below? Can you share your idea? [root@hosting ~]# exigrep Rammy.Naguiat@westernunion.com /var/log/exim_mainlog 2018-10-14 23:39:11 1gBiU2-0002Lu-6U U=mailnull Warning: "SpamAssassin as cpaneleximscanner detected OUTGOING not smtp message as NOT spam (0.0)" 2018-10-14 23:39:11 1gBiU2-0002Lu-6U <= <> R=1gAnLk-00070V-Hd U=mailnull P=local S=20237 T="Mail delivery failed: returning message to sender" for prvs=7823b7baca=rammy.naguiat@westernunion.com 2018-10-14 23:39:11 1gBiU2-0002Lu-6U Sender identification U=mailnull D=-system- S=mailnull 2018-10-14 23:39:11 1gBiU2-0002Lu-6U ** prvs=7823b7baca=rammy.naguiat@westernunion.com R=dkim_lookuphost T=dkim_remote_smtp: all hosts for 'westernunion.com' have been failing for a long time (and retry time not reached) 2018-10-14 23:39:11 1gBiU2-0002Lu-6U Frozen (delivery error message) 2018-10-15 01:14:12 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 01:15:35 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 02:02:01 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 02:55:43 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 03:02:27 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 03:03:12 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 06:42:15 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 07:09:27 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 07:10:15 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 07:17:46 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 10:52:37 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 11:08:17 1gBiU2-0002Lu-6U Message is frozen 2018-10-15 17:31:47 1gBiU2-0002Lu-6U removed by root 2018-10-15 17:31:47 1gBiU2-0002Lu-6U Completed 2018-10-15 00:12:45 1gBj0W-0007Id-0m U=mailnull Warning: "SpamAssassin as cpaneleximscanner detected OUTGOING not smtp message as NOT spam (0.0)"