use warnings;
use strict;

my $str1 = 'Oct  9 08:50:53 mail_server sendmail[30172]: j99FopoN03017
+2: from=<support@symantec.com>, size=0, class=0, nrcpts=0, proto=ESMT
+P, daemon=MTA, relay=[218.1.114.182]';
my $str2 = 'Oct  9 04:20:35 mail_server sendmail[20773]: j975sOqf03231
+2: to=<522-2204194-1-13-51954mpxas@dimexpress.com>, delay=2+05:26:11,
+ xdelay=00:00:00, mailer=esmtp, pri=19293815, relay=mail.xpress.com.'
+;

my $match = qr/relay=\[?([^\]]*)/;
print "$1\n" if $str1 =~ $match;
print "$1\n" if $str2 =~ $match;
[download]

Prints:

218.1.114.182
mail.xpress.com.
[download]