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If I was hosting emails for 3 clients and my reverse DNS is mail.allclients.com and I am hosting for client2.com, client3.com etc. What would be the rDNS for client3.com? Would it reflect the rDNS of mail.allclients.com? If so, I do I prevent mails from client3.com from being blocked because the PTR does not match rDNS? Finally on your advice to have my email service publish an SPF record. Is this something I need to submit to the company I registered my mail.allclients.com to? So I for example registered with GoDaddy.com then I will need to submit a request to them to publish an SPF record on their DNS right? Thank you for your advice and kind assistance.
Vince Saavedra

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Reverse DNS is based on the IP address. You can have many forward DNS names resolve to one IP address, but that one IP address should only reverse to one DNS name.

Most email real-time black lists are also based on IP address. If your IP address is black-listed or has a poor reputation, you will need to make sure no spam is being sent through your server and contact the individual list maintainers for help getting de-listed. Since you are hosting for clients, you will need to instruct those clients carefully on what is Acceptable Use and act quickly to block anyone violating that.

Adding an SPF record to your DNS record does not hurt, provided you keep it up to date. It is good practice to have an SPF record, but mostly because it slightly reduces the ability of other people to impersonate your server. It does little or nothing to improve your own server's perceived reliability. Whoever is maintaining your DNS would need to create the SPF record.

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