I just found out that LunarPages' cPanel tool to generate a SPF record (Email - Authentication - SPF) is broken and leading to your emails being rejected.
cPanel "appears" to generate a valid SPF, automatically including the IP of your server.
What LunarPages hasn't told anyone and you would only find out by looking at the headers of your email that actually got delivered is that your server no longer delivers outgoing emails directly. Lunarpages has added an intermediate server, outbound-lv01.lunaremail.com in our case, that does the actual delivery. Since this server isn't in the SPF record, filters declare the emails as coming from a fake server and reject it.
You have to manually figure out what server your emails ACTUALLY go out through and manually ADD it to the SPF record.
This can also cause ALL your domain's outgoing emails to be blocked by Lunarpages. Because of the SPF mismatch, the destination servers may also bounce the emails back to you. If there are more than 5 of these in a rolling hour Lunarpages will shutdown all your domain's outgoing emails for at least an hour. This is what they call a deferred and bounced filter to catch spammers trying to send FROM Lunarpages. In fact over half of the situations that can trigger this block are not signs of a SPAMMER and not under the sender's control. Such as: destination server down, destination server too busy, destination mailbox full, destination network unreachable, destination address has been discontinued recently, Lunarpages' broken SPF/email service, DNS failure...