how do I receive mails in appian ?

Then please check the available documentation. We will be happy to help when you have more specific questions.

https://docs.appian.com/suite/help/21.1/Send_Email_Smart_Service.html

docs.appian.com/.../Sending_an_Email_Message_to_a_Process.html

We are receiving a file in email and we need to import the data of that file into appian table. How can we read the file received in the mail and load the data from file into appian table

That depends on the type of the file and there are OOTB smart services and some plugins to import data. Please check the documentation and the app market for plugins.

That depends on the type of the file and there are OOTB smart services and some plugins to import data. Please check the documentation and the app market for plugins.