Have them check their spam folders. If it's in the spam folder then GMail has blacklisted you as a spammer. There isn't much you can do about getting off GMails blacklist except email them your mail server IP Address and your domain then ask them too, but it took us over a year to get off their blacklist so it's basically a nightmare if this is your case. Alternative is to use a GMail account using SMTP to send your emails, but you're limited to like 1,000 per day or something of the sort.
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Gmail has its issues from time to time - see this thread
productforums.google.com/forum/#!topic/gmail/rSan4jwPlGQ
so it could just be that gmail itself is having a problem especially as your users with other email hosts are fine.
Worth checking that your domain has DKIM (Domain Keys Identified Mail) set, which generally helps to qualify the email traffic produced by your website.
krileon
