In SiteInformation I defined E-mail address as "info@web.mydomain".
It should be used as "From" address in automated emails sent during
registration. But it is simply not used in any emails...
Instead of that, wen user registers, he gets and email where "From"
field has value "Apache <apache@fqdn.of.my.server>"
So what am I doing wrong? Why is drupal not using pre-defined
email address for "from" field in emails?
It may the behavior of your hosting provider to suppress the from e-mail address, if it doesn't exist. I had this problem earlier with bluehost. Replacing "info@web.mydomain" with some other account that exists in your server may help.