Before we start...
An email address is composed of a local part and a domain: [local part]@[domain].
For which conditions an email or domain would be flagged as spam?
If the local part OR domain is exactly “test”
If the local part OR domain begins with “test”
If the local part contains “+test” anywhere
If the local part has “test” followed by any number of digits, and that’s the end (ex: email@example.com)
If the domain contains “.test” anywhere
Some characters are repeated many times
If any character is repeated at least 4 times consecutively
If any pair of letters is repeated at least 4 times (firstname.lastname@example.org)
Numbers exceed letters
If the local part is made up of more than half by digits (email@example.com)
If there is at least one more digit than there are letters in the local part
If there are at least 6 numbers in the local part
Local part has no letters
If the local part does not contain any letters
Spammy patterns detected!
If the email is longer than 20 and the top 3 most common characters make up more than 70% of characters
Domain end is domain
If the strings before and after the period in the domain are the same (firstname.lastname@example.org)
Domain or local part contains backlisted word or phrase
If the domain contains any phrase from an in-house list of blacklisted words/phrases
Contains asd or sdf twice
If the email contains the sequence of letters “asd” or “sdf” twice
Looks like spam
contains the phrase “noemail” anywhere in it
Local part or domain length is 1
the local part contains contains one character (email@example.com) or domain contains exactly one character (firstname.lastname@example.org)
Local part has no vowels
If the local part of the string is at least 4 characters long, does not contain any numbers, and does not contain any vowels.
Local part low vowel ratio
If the local part is 5 characters long and there are no vowels in it
If the local part is greater than 5 characters and the fraction of vowels is less than 0.08 (specifically, vowels / letters, not vowels / total number of characters. As in, numbers are not being counted when doing the division)
Domain contains short gibberish
If the domain contains any of the following:
If the domain looks exactly like any of the following:
If the local part contains any of the following:
If the local part is exactly:
Not F1000 or personal and has numbers in local
There are two consecutive numbers in the local part.