MEDIAWIKI has some functions to send emails, for example you can have MEDIAWIKI mail you a new password.
You have to set $wgEnableEmail = true; in LocalSettings.php to enable that function. By default the standard PHP mail function is being used, whatever that means, it did not work for me - I assume this is due to the fact that I have neither Sendmail nor Postfix running on my Linux server.
In LocalSettings.php you can set
$wgSMTP = array(
 "host" => 'myhost.mydomain.myTLD',
#  "IDHost" => 'domain for MessageID',
   "port" => "25",
    "auth" => false,
#     "username" => user,
#      "password" => password
      );

This will use PEAR Mail which is not included with MEDIAWIKI. On my SUSE distribution I used yast and found that "PHP Extension and Application Repository" - "php5-pear" was already installed. I was not able to start pear using "pear" in a shell but "pear5" did it.
Now I was able to install the mail component by using the pear package manager:
pear5 install Net_Socket
pear5 install Auth_SASL
pear5 install Net_SMTP
pear5 install Mail
Another test in my WIKI still failed because Mail.php was not found.
I had to edit LocalSettings.php again to change
ini_set( "include_path", ".:$IP:$IP/includes:$IP/languages" );
to
ini_set( "include_path", ".:$IP:$IP/includes:$IP/languages:/usr/share/php5/PEAR/" );
but the path may vary for other distributions.

The last part is to exclude the IP-Address of the WIKI server from all anti-relay options in Lotus Domino.