When validating your email list with MailboxValidator, you might have noticed a field called is_greylisted. If the value for the field is True then the mail server for that email domain has greylisting enabled.
Greylisting is an anti-spam mechanism employed by some mail servers. When you send an email for the first time to a mail server that has greylisting, a 4xx error code is returned, indicating a temporary rejection of the email. After a certain period of time, you can resend that email and it will be accepted. The period of delay depends on the configuration of the target mail server which could range from minutes to days.
Works well against spammers
This works against some spammers because the scripts or programs used by spammers will not retry the sending of an email. Meanwhile, normal mail servers will queue up the rejected emails to try and resend later. They will often retry the sending of the email multiple times for a few days before giving up.
Time-sensitive emails will have issues with greylisting
Greylisting might work on spam but unfortunately it also affects normal emails. Lots of websites these days will send emails with One-Time-Passcode to login to their websites as part of their 2FA security measures. Very often, these codes are only valid for a few minutes or at most an hour. If the greylisting delay is longer than the code validity, the user will never be able to complete their login routine.
How can I send email to a greylisted address?
For emails that are not time-sensitive, most mail servers will queue up the rejected email and retry later. Assuming you are using a properly configured mail server, this shouldn’t be an issue. Your recipient will just get your email a bit later than expected. In this case, you might want to inform the recipient about the delay.
If you send an email where time is of the essence, then you will need to contact the recipient to get them to whitelist your sender email domain. Most greylisting systems accept whitelisting of email domains. So, all of your emails should reach the recipient without any delay.