I am running a WHMCS hosting system for a small number of private clients. For a few of them they never seem to receive the invoice emails I send out. I have ran the emails through a number of tests such as sending one to firstname.lastname@example.org and when I get the report back it says:
Summary of Results
SPF check: pass
DomainKeys check: neutral
DKIM check: pass
Sender-ID check: pass
SpamAssassin check: ham
Yet like I said some clients still never receive their messages while others get them with no problems. If I send them a direct email from my personal account they get it just fine. Most often it is Google Apps users that never even see the email go to their spam folder. The email just never arrives...
Any idea of what else I should check to see if there are any problems that might be making my invoices dissapear?