The primary blocker of email is spam filtering. We use SPF records on area101.com to indicate what email servers on the internet can send mail on our behalf. They look like this:
$ dig -t txt area101.com
area101.com. 3600 IN TXT "v=spf1 mx include:spf.protection.outlook.com include:spf.area101.com ?all"
The above records tell any email servers who receive an email from area101.com that the email servers at outlook.com (as defined by spf.protection.outlook.com) and area101.com (as defined by spf.area101.com) are the only permitted senders, and that any other source of email is dubious and should be blocked. The alternative is to specifically whitelist our sending ip (currently at mx1.area101.com).
We define permitted senders of area101.com emails as 3 ranges of IPs where our servers reside, where we also send FoodVenue emails on behalf of other domains.
$ dig -t txt spf.area101.com
spf.area101.com. 3600 IN TXT "v=spf1 a ptr ip4:18.104.22.168/28 ip4:22.214.171.124/28 ip4:126.96.36.199/28 ~all"
So, to ensure proper delivery of our FoodVenue emails, our SPF record should be included as "include:spf.area101.com" per the first example above, for each domain. Alternatively, they may whitelist our mail server IP from mx1.area101.com, or whitelist all 3 IP ranges in the second example above.
Short summary for IT folks:
Add include:spf.area101.com to your SPF record or whitelist mx1.area101.com. If you whitelist, consider whitelisting all 3 ip ranges above.