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:
18.104.22.168 to 22.214.171.124
126.96.36.199 to 188.8.131.52
184.108.40.206 to 220.127.116.11
18.104.22.168 to 22.214.171.124
126.96.36.199/21 and 188.8.131.52/22
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.