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| Email also requires the DNS server(s) ip addresses as well. | |
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Only if you use the server name - if you put in the IP address directly, you don't need DNS, but it's not worth it.
I highly recommend you use DNS and the server name (i.e. smtp.companyname.com) in case the ISP swaps out a server. Typically they will bring both servers up on different IP addresses, change out the A record in DNS, give it a while for everybody's DNS cache to update, and shut down the old server once everything resolves to the new address. This lets them upgrade the server with absolutely zero downtime, but mail now goes to a new IP address. Works perfectly unless somebody hardcodes an IP...
When a major ISP upgrades their servers you'll suddenly have dozens or hundreds of machines that suddenly can't scan to email.