MailDomainRestriction is a mediawiki extension restrict email's domain at account creation.
Extract extension ant place it in the 'extensions' directory of your installation. (the directory namme must be 'MailDomainRestriction')
Load extension in file LocalSetting.php :
wfLoadExtension( 'MailDomainRestriction' );
and in this file, add either a domains whitelist or a blacklist :
$wgMailDomainRestrictionWhitelist = ['alloweddomain.com'];
or
$wgMailDomainRestrictionBlacklist = ['forbiddendomain.com'];