An e-mail could not be sent because the run as user for this activity could not access one or more of the groups listed in the e-mail address.
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Hi Lokesh, It looks like the user who has started the process does not have sufficient privilege to access the groups. Please check group's visibility
The Error is observed while creating the group if you have selected Only members and Admin can view and user is not part of the group for which the mail is being sent, Go to group properties and make it public to resolve the error or add user to that group