Hi Pratyush,
I also faced similar issue. This is because, you might be using Client's mail SMTP server. The outlook mail ID might be your company's domain, as well sending email ID also might be your same company domain email ID. If this is the case, then this is a Compliance issue.
Solution is to change the sender email ID to any other domain apart from your company's domain. It can be any dummy email ID also. Check if this sends mail to outlook.
Hope this helps.
Regards-
Chirag Keswani