Hi @pavan kumar
Agree with Andy, I'm afraid you will have trouble sending mails to external.
You could also refer to the discussion in this related thread: How do you send an email within a network without internet?
You will not able to send email from a source outside a network to a device inside a network when you've cut off the connection between the outside and inside.
However, devices on an isolated network can send email to each other.
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