how can i create and maintain internal email server free ..please suggest me any free downloads ...

If you already have Microsoft Exchange server (as indicated by your Topics selection), then you should already have an internal mail server ready to be configured.  

If not, please expand your question detailing what your network setup is?  Server information etc.

Also, what do you mean by "Office of 200 seater"?

You won't be able to host mail servers in 200 -/+ PCs for individual mail addresses pertaining to each user who sits in that seat..  You need a central server to handle the incoming and outgoing mail, plus process the 'bounces'.   Perhaps try Google G suite hosted mail or Office 365, Exchange Online.. but nothing for 200 people is free.. that I know of.

You could build a Linux box for the price of the hardware and host the mail yourself, but you'd need to know about Hoard or other mail handlers and you would need to know a bit about Linux... and firewalls.

Well you have 2 options...either setup a linux box as email server ...Ubuntu+Zentyal (its a good easy to configure ) or use the free HmailServer which is a free windows application ....

Have a look at Axigen:

https://www.axigen.com/mail-server/free/

Pease bear in mind, free solutions are great, but what happens when theres a problem? Do you have the skill, (or if the skill's available) to repair the email server, or recover your mail from it in the event of a disaster scenario. If you go for a Linus solution what are the in house Linux skills like?

Best of Luck  

Pete

Linux skills is always the big question mark when free open source Linux based solutions are proposed.....i still recall my horrendous experience with such a free solution a decade ago...countless hours reading adjusting and again reading, trying, setting....then one day came the paid (windows) solution...i just installed it and that was all....ten years and is still working....
Side Note : I like Linux and i have my own private cloud Linux server for all my development needs but....

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