Just curiously, is there any compelling reason to host your own SMTP server ? There is no need to host your own SMTP server within your premises. You can always colo host one in data centers, or you can also make use of AWS SES as your outgoing SMTP server, or buy a Google App service.
I could be mistaken when you mentioned email server. Are you referring incoming SMTP server for your domain, or are you referring to outgoing SMTP server to send out emails ? I was more referring to outgoing SMTP server in my previous response to you.
However it is common to find incoming and outgoing SMTP servers are hosted under the same MTA and used for dual purpose.
Actually I just want to set up a mail server for own use such as own email account instead of using gmail etc.Didnt know it so complex. Thanks for explaining but I really do not understand the difference between incoming or outgoing smtp server.