First, check your spam folder. Sometimes spam filters will catch emails that appear to come from your domain but have been sent by a third party. Normally this is a good thing, but in this case it might keep you from getting your preview.
If your preview isn't in spam, make sure that your email server isn't blocking our emails. Whitelist the following IP address ranges:
220.127.116.11 to 18.104.22.168
22.214.171.124 to 126.96.36.199
188.8.131.52 to 184.108.40.206
220.127.116.11 to 18.104.22.168
22.214.171.124/21 and 126.96.36.199/22
If you still are not receiving preview emails, try sending it to another email address in your same domain. If it goes through, try sending it to yourself one more time - perhaps there was a misspelling or something similar.
If all else fails, please contact Lead Liaison Support. They can verify if the email is reaching your server and do additional troubleshooting on our end.