Hi,
Sendmail is really the first of serious SMTP MTA programs. In fact it is just a chicken and egg story when it ocmes to SMTP and Sendmail. It is the most popoular of the global MTS and it had a share of more than %40 in 2005 and there were 4 other rivals each had oony around %10 share.
It is one of the very popular and widely used open source programs. It is flexible yet it is kind of hard to configure it especially people with no previous knowledge of Sendmail configuration.
In fact when it comes to comparison with Notes, Exchange and other Groupware software just implements a portion of what Sendmail provides and it is performed through 1 or 2 web page full of options.I've used and administered both Domino SMTP and Exchange. They implement SMTP as a side program to support their Groupware functionality.
When I have to suggest some robust SMTP gateway to my clients I generally suggest them to install a Unix / Linux SMTP gateway between the internet and the corporate email server whether it is domino or Exchange. This gives more flexibility and security.
As it is the original domain that your question belongs sendmail comes default with IBM AIX and most Linux distros around. But I personally find it very difficult to configure and switched to Postfix 5 years ago and never regretted even once since.
Postfix is another MTA created by then an IBM employees and it offers direct compatibility with sendmail and easy to manage configuration files.
As you know UNIX is an operating sytem and it may or may not need to run an SMTP daemon so they are not related although sendmail is very common over UNIX.
Sendmail already an SMTP server an that is it. It has nothing to do with the OS it runs over.
Sendmail is used because the OS supports by default (as in the case with AIX) or because the admin is most likely be familiar with it. But as I told you earlier I personally prefer postfix over the sendmail due to configuration flexiblity.
Cheers,
K.
