Exchange 2013 – DNS Records for Accepting Multiple Domains

There are several things wrong I can see here:

1. Just so you're clear, a DNS domain on the Internet is not necessarily the same domain you're using in Active Directory. It can be the same, but more often than not (in smaller organizations), you'll choose to use a domain such as centors.local* that's only applicable/resolvable internally by your Active Directory DNS servers.

   Your Internet domain is usually (and really should) be handled by an external DNS server (and typically your domain registrar or a 3rd party company that specializes in DNS like no-ip.com or dnsmadeeasy.com). This is because:

   a) there's usually no beneficial reason for handling your own public authoritative DNS server unless you're a large enterprise or have atypical/legacy reasons to do so; I'm confidently going to say that you don't have any of these requirements.

   b) managing your own Internet facing DNS server is not easy: you need to make sure it's always up, has geographical redundancy (multiple servers, in different physical locations) and is secure (DNS poisoning and amplification attacks ad naseum).

2. You likely don't need to change your Nameserver (NS) records. These are used to tell the world what DNS servers are authoritative for your Internet domain (centors.com). When you register a domain at networksolutions.com, godaddy.com, etc. they typically offer authoritative nameservers for you (and thus have already assigned their NS records to your domain), so all you need to do is log in to your DNS control panel and modify the records (the zone). Typically a small business will only ever need to create/modify A, CNAME, MX, and possibly TXT (for SPF) records.

3. To get your mail flowing properly, you need to create an A record and an MX record: typically the A record would be mail.centors.com, and this A record would resolve to the publicly-accessible IP address (assuming 41.233.26.131) that your Exchange Server is either NAT'ed behind or assigned itself (usually the former as the Exchange Server will be on the LAN behind the router/firewall). For the MX record, you would create an entry with a number to denote order of priority. Since you only have (I'm assuming) one mail server, you would just add mail.centors.com with a priority of 5 to your MX records.

4. As for Active Directory authentication issue, I'm assuming that you haven't configured POP3 yet on your Mail Server. I would strongly recommend you don't add POP3 support unless you have some legacy reasons why. It's a terribly inefficient way of handling mail, especially if you have an Exchange Server. If your Outlook 2010 client is on the LAN (and is on a workstation that's joined to your Active Directory domain), you want to add an Exchange account as the mail type, not POP3.

*As for your DNS in Active Directory, I'm not sure how far you've gotten with it, but I would recommend you change your Active Directory domain to something that you won't use publicly like corp.centors.com or centors.local (the latter is not recommended by Microsoft because it does not guarantee that the domain is unique and if you ever merged with another "centors" you would need to change your domain; it's up to you whether that's a risk or not).

I have now found the issue - everything was working as expected; however Outlook had cached the email address I was sending a test to, and causing the issue. Deleting my .NK2 file fixed this.