The Pre-Processing analyzes your data and carefully screens for such problems that can produce misleading results. Reporting on the quality of data before running the email validation analysis.
The format of email addresses is local-part@domain where the local-part may be up to 64 characters long and the domain name may have a maximum of 255 characters however the maximum of 256-character length of a forward or reverse path restricts the entire email address to be no more than 254 characters long.
MX Record (Mail Exchange Record)
This is the DNS record that specifies the mail server addresses used to receive incoming mail for a specific domain. Typically, at least two mail server addresses are provided for a primary and alternate means of transmitting an e-mail to an address "@" your domain.
Since usually more than one mail server address is configured for use with a mail system, it is necessary to set priorities when entering the addresses. The priority specifies which address should be tried first to handle mail sent to that domain. If the primary mail server address would become inaccessible or malfunction, then the second mail server address with a lower priority would then be tried.