Hi @Prashant Adhikari, 

Thanks for your post! As a first step, could you double-check that all of the IP and email addresses in this Lucidchart doc are allowed-listed on your end? This will help us rule out issues with your email client blocking emails from us based on the IP or sender.

Quick tip : If you’d like to copy those addresses and paste them elsewhere, navigate to File > Duplicate to create a duplicate of the doc. You will then be able to select and copy text from that duplicate.

Once you allow-list those addresses, please resend the invite to the users by navigating to your Admin Panel > User tab > Pending Invitations. From there, select the checkbox next so the user you want to invite > click the three dots > select  “Remind User” to send them another invitation email.
 

Could you try these steps and let us know how it goes?