I have a mailhost settings as firstname.lastname@example.org (name changed) , from splunk alerts mails are only going to @abc.com, but our requirement is to also send mails to @bdc.com.
What are are changes required to send mails to multiple domains
That looks more like a mail gateway configuration at your mail host. Splunk has only validations on the domain in restmap.conf but since both of your domains are 'com' - that should not take effect. Check with your mail administrator if there are any domain restrictions.