The DNS record type that the SMTP server looks for is called as MX records.Once your SMTP server finds the target receivers SMTP server, using an MX record lookup, it will forward the message to that server (through the internet ).So if you want to send an email to somebody, you need to talk to that person’s SMTP server(Basically your domain's SMTP server will talk with the receivers SMTP server while sending an email).Below mentioned are typically the steps that happen while sending an EMAIL.
The Domain Name System(DNS) has several types of resource records that full fill the name to ip address translation(Directly or indirectly).
If its not part of the same zone (for example, you have given your mail server as something like mail.or mail.anything.com), then that name should be resolvable to an IP address (A record) externally. This enables you to modify your high priority mail server yet again, if something happens to the earlier one. ns1 IN A 10.0.0.23 mail IN A 10.0.0.24 mail2 IN A 10.0.0.29 www IN A 10.0.0.27Another technique that you can do with DNS to load balance your mail server is to have multiple servers with different IP addresses, accepting mails for the domain.
All this can be done without touching any other MX record. Say for example, you have two mail servers 10.0.0.33 and 10.0.0.34, which both can accept mails for domain.
An email client is sometimes called as a MUA(Mail User Agent).
You will often encounter this term, when discussing about e-mails.