One of my business customers was having a problem this morning with emails being returned that had been sent from their email at HostGator. After researching the problem, I discovered that the emails being returned had been sent to Yahoo.com and Aol.com email addresses.
I contacted HostGator and they told me 
they had been greylisted by Yahoo.com and Aol.com. This means that Yahoo and AOL are blocking any emails that are coming from Host Gator mail servers and they won’t be delivered.
They weren’t sure when this problem would be resolved, but they were working on it and were aware of the situation. They also didn’t tell me why Yahoo and AOL had greylisted emails sent through HostGator.
If your website is hosted by HostGator and you’re using their servers to send emails, be aware that all yahoo.com and aol.com emails will bounce until HostGator gets the greylist removed. Many of the emails you send out aren’t shown as undeliverable until the next day or at least several hours later.
So if you’re invoicing customers, or sending out important emails to people who have Yahoo or AOL email addresses, they won’t receive them until HostGator gets this issue resolved.
I Googled this to see if anyone else had this problem, and discovered that off and on for a few years now, HostGator’s customers have been complaining that emails aren’t being delivered if sent to Yahoo or AOL email addresses.