I have problems with receiving emails from forum. I forgot password and tried to recover it but didn't receive email. I looked in my mail-servers log and found that it is rejected. Reason: Helo command rejected: Go away (dynamic) helo=<ip-172-31-0-229.ec2.internal>. It's bad helo because many SMTP servers filters out this as spam from dynamic addresses. I was forced to register from gmail what apparently uses more advanced spam filtering methods and allows such emails.