Backup MX Server - verify my setup + spam
I have a vps for testing, learning and hosting some of my sites, while I have reseller accounts with other providers (I host my clients' sites). A couple of days ago I set up the vps as a secondary/backup mx for the other domains hosted on resellers accounts.
The only changes I made on vps were :
a) added the domains I wanted to accept emails and then resend at /etc/secondarymx
b) I created an A entry like mailstore1.myvpsdomain.com
Q1 : Is there any other change to be made on vps? I sent a test email from vps and the mail was delivered to remote domains on resellers' account.
Q2 : How to test/verify if the mails will be delivered if the primary mail server is down?
Q3 : Do I have to create on vps an A entry with the IP of the reseller accounts?
Also, these days I noticed in the vps mail queue some emails were there trying to be delivered for many hours, all of them were spam and some were addressed to non valid emails. I've read that spammers search for the secondary mx entries to send spam as they might be act as open relays, but I use antirelayd.
Q4: How they manage to send their spam directly from the secondary mail server bypassing the primary?
Q5 : Is there a way to avoid this? I knows these message are not going to be delivered but if they are lots they will probably going to cause a high load on the vps.
Thank you for any input and help !
