Reply checks each email address by different parameters. Depending on the results of the check, the system marks email addresses as Valid, Risky and Invalid. Below is the explanation of each status.
Valid
Valid email. Verification was completed to mailbox level. No high-risk factors detected. Safe to send mail.
Risky
The Risky status means the recipient's mail server stated the recipient exists but other factors indicate this address may still result in a bounce, or is of low value. We recommend treating the Risky status as Valid. Best practice is to assign no more than 20% of Risky emails per sequence in order to avoid high load of bounces and sequence suspension. We consider the email address as risky in the following cases:
Domain is a well-known DEA. The email address is provided by a well-known disposable e-mail address provider.
Catch all. The external mail exchanger accepts fake and non-existent e-mail addresses; therefore the provided e-mail address MAY not exist. The existence of the individual mailbox cannot be verified.
Honeypot. The external mail exchanger hides a honeypot / spam trap.
Invalid
The email address is considered as Invalid, if the response contains one of the following errors:
Syntax errors
A quoted pair within a quoted word is not closed properly.
An unexpected quoted pair sequence has been found within a quoted word.
A new word boundary start has been detected at an invalid position.
An invalid character has been detected in the provided sequence.
The number of parenthesis used to open comments is not equal to the one used to close them.
An invalid sequence of two adjacent dots has been found.
The local part of the e-mail address has an invalid length.
An invalid folding white space (FWS) sequence has been found.
The at sign symbol (@), used to separate the local part from the domain part of the address, has not been found.
An invalid quoted word with no content has been found.
The email address has an invalid total length.
The domain part of the email address is not compliant with the IETF standards.
DNS errors
The domain of the e-mail address does not exist.
The domain of the email address does not have any valid DNS records and couldn't accept messages from another host on the Internet.
Mailbox errors
The mailbox for the e-mail address does not exist.
The external mail exchanger rejected to accept messages.
The external mail exchanger does not support international mailbox names, while the supplied address pretend to have one.
Note
A contact with the Invalid status can be moved in a sequence if there are no email steps at the moment. Such measure helps handling large volume of contacts, limits and deliverability. This covers API as well.