I have two forms FormA & FormB. 

In FormA there are field called Email & Country(Multi Check box)

In FormB there is field called Email (Lookup to Email field in FormA) & Countries (Dropdown box)

What I wanted to achieve is, when an email is selected in FormB, field Countries should populate the list of values we selected in FormA for that email.

Please let me know if you need more clarification.

Thank you in advance for your support.