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Where are you trying to receive the Emails? If you are trying to use an external email service you want the MX to point to them if you are trying to use the built in Email Accounts with InfinityFree it will need to be pointed to mx.epizy.com.
Yes, you can use any external email service as long as you point the MX to them. If the Email Hosting provider allows POP, IMAP and SMTP you will be able to use the plugin "SMTP Mailer" to email using that provider. Link to plugin: https://wordpress.org/plugins/smtp-mailer/