Apple

smtp;421 4.7.0 me.com Error: too many errors

smtp;421 4.7.1 Messages to example@icloud.com deferred due to excessive volume. Try again later - https://support.apple.com/en-us/HT204137

Medium severity

smtp;451 4.7.1 Service unavailable - try again later

Low severity

smtp;550 5.1.1 <example@icloud.com>: user does not exist

smtp;550 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137

smtp;550 5.7.1 Your message was rejected due to example.com’s DMARC policy. See https://support.apple.com/en-us/HT204137 for info

High severity

smtp;550 5.7.1 Your email was rejected due to having a domain present in the Spamhaus DBL -- see https://www.spamhaus.org/dbl/

Critical severity

smtp;552 5.2.2 <example@icloud.com>: user is overquota

smtp;554 5.7.1 <example@icloud.com>: Relay access denied

smtp;554 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137

Medium severity