<?php
$str = 'txt stuff
Administrative Contact:
Brown, John support@xxxxxxx.com
txt stuff';
preg_match('~admin[^:]*:\s*(.*)\s+\S+@~i', $str, $matches);
PD9waHAKIAokc3RyID0gJ3R4dCBzdHVmZgpBZG1pbmlzdHJhdGl2ZSBDb250YWN0OgpCcm93biwgSm9obiBzdXBwb3J0QHh4eHh4eHguY29tCnR4dCBzdHVmZic7CgpwcmVnX21hdGNoKCd+YWRtaW5bXjpdKjpccyooLiopXHMrXFMrQH5pJywgJHN0ciwgJG1hdGNoZXMpOwp2YXJfZHVtcCgkbWF0Y2hlc1sxXSk7