I work in small size company and for mail server my chief 
says MS exchange is little baby.
I saw MS can make a good product but big is question why older administrators like to use sendmail which doesn't have calendar.
Is sendmail alfa and omega in mail servers ?
First and most obviously, these are the UNIX forums, and Exchange is a Windows product.
The UNIX philosophy is that programs should do one thing and do it well. A mail server should be a mail server -- not a bloated all-singing all-dancing time management monstrosity. Go down that path too far and you get Lotus Notes, a mail program so baroque that nobody can tell you what it actually does in full, or why, without beginning to drool and twitch.
There's a lot of good reasons to not want to use Microsoft products, besides. The upgrade treadmill is one. The ridiculous prices are another, even as we watch licenses grow more and more expensive plus less and less useful. The long history of legendary security goofs, and continuing security issues even now, is a third.
As an aside, vanilla sendmail is pretty obsolete now. There's other compatible options like postfix.
I am an older administrator who still uses Sendmail. I have about 20 years experience with it and I can solve any Sendmail problem I encounter. It was a nasty learning curve and I really don't want to do that again. Sendmail has had more than its share of problems over the years but now it seems pretty stable. It would be possible to switch to postfix but I don't see any reason to do that. Exchange is not even a possible option as far as I know... it doesn't run on Unix. Postfix is the only alternative I know... are there other MTA's available for Unix?
I have to say we Sendmail users probably should not chide Exchange on its security problems though.... people who live in glass houses shouldn't be throwing stones. 
First of all, your question is valid, and you can ask questions about MS Exchange in our forums, and are encouraged to do so.
Yes, these are The UNIX and Linux Forums, but that does not mean we do not welcome those who are using other operating systems.
Personally, I prefer UNIX-based mail systems because I prefer open systems to closed systems; but it would be great to see an excellent UNIX/Linux mail system with integrated calendar.
In fact, when I used to consult for many large companies, the single biggest reason they used MS Exchange was for the integrated calendar functionality. Business people like their email and their calendars tightly connected, and this has been a great success of MS Exchange (in addition to their LDAP implementation).
In closing, I would like to remind everyone that we don't simply bash Microsoft or other operating systems in these forums and it has been a long standing violation of our forum policy to do so.
Yes, MS Exchange might be "bloatware" to some; but for countless large businesses, including most of the largest organizations in the US, MS Exchange, with Calendar and LDAP integration, is the backbone of the business operations. What is important is what "users" like, not want "older system admins" (like us) think
In my previous company when I worked we used microsoft exchange. Why? Because the business people understand how to use it and prefer it and ease of functionality. Some things unix are very good at like sending massive bulk emails, I've seen massive 900k emails going out no problem, we did a test on a mail product running on ms which crashed on a load of less than 100k mails.
But some things a unix mail server cannot do like setting your own vacation message. How do your business users who are usually non-technical babies set up their own vacation messages or change passwords without having to telnet or ssh to the server
In one of the other companies where I worked we used unix smtp and pop3 a very simple setup running off an old ultra250. But we often had business people coming to us all the time to set up their vacation messages and asking us to change their passwords for them. It was a small company with less than 30 employees, it was rather informal place & they were not very big on security so this practice quite acceptable but in big organizations like the one described above where I worked we had like 300,000 employees worldwide such a setup and practice will not work or be accepted.
So I guess there's no 1 size fits all out there. Different products are good for doing different things.
I think Coronas opinion has some truth to it and I'd like to expand on that:
The UNIX philosophy is indeed to have every tool do one thing and be good at it. To quote from my favourite RFC (1925):
It might be good to have the calendar and the mail integrated, but the downside is that Exchange can't be brought to work as a real MTA. There are so many inaccuracies in the implementation that the only client it really works with is Outlook. I don't think that anybody in his own mind wants to use Outlook as a mail client. Outlook will outright refuse to show you the email address of a local recipient (it will - whatever you do - change "john.doe@yourcompany.com" to "Doe, John"), it cannot be brought to format replies correctly, it will send mails HTML- (or even worse) formatted instead of in ASCII even if you configure it not to do so, and so on and so on.
In my current project i am forced to use Outlook 2007 (means extra punishment: this damn "Fluent" GUI can't be deactivated and the look and feel of the OS and the application is forced to be inconsistent) and after 2 days trying to configure this thing to work like a mail client is supposed to work i simply gave up, write my mails with vi and a bunch of vi macros and copy/paste the results to/from the Outlook window. Sorry, but this is not "user friendly", this is inacceptable.
The LDAP implementation is also quite buggy: we set up OpenLDAP as general authorization tool across the the "open systems" (that means AIX, a few HP-Ux- and SunOS-boxes and [CentOS-]Linux here) and my colleague wanted to connect the Exchange Server too: no chance, somehow it's LDAP, but not quite - this is, sad to say, often the case with MS products. They claim to implement a standard but implement it in a way that only their own products understand it.
bakunin
sendmail contains a flexible rule engine. It can do more than just handle mail. Not saying that sendmail needs to stick around, just saying you can do a lot with sendmail because of this "generic" piece. It may be a bit cryptic, which is why you have things like "mc" files and such, but if you DO understand the macros and rule engine of sendmail, you can do weird and wild things with it. Just saying...
I have worked for some of the world's largest organizations and most all of them use MS Exchange, primarily for the calendering and scheduling function, which they consider essential to their business operations. This demonstrates that the choice of technology is mostly user-driven, not system admin-driven, in larger organizations; and I disagree with most of what others said against Microsoft and the harsh words against MS and name calling against MS Exchange as a product, and I gently reminding all about our forum rules, which have been in place for a long time, which I will attach in another post.
Hello!
In case you forgot to read the forum rules, here is quick copy (and a few highlighted, for added emphasis).
Cheers.
The UNIX and Linux Forums
Please guys don't transform this topic in holly war. This forum is for Linux but I think we can discuss about technology and exchange experience in polite manner.
Who started the holy war? I don't see one.
This thread is not doing anyone any good, so I think it's time to close it.