Reasons things could be marked as spam:

is test

If the local part OR domain is exactly “test”

OR

If the local part OR domain begins with “test”

OR

If the local part contains “+test” anywhere

OR

If the local part has “test” followed by any number of digits, and that’s the end (ex: logantest333@madkudu.com)

OR

If the domain contains “.test” anywhere

not F1000 or personal and has numbers in local:

The domain type is not fortune 1000 or personal and there are two consecutive numbers in the local part.

numbers exceed letters:

If the local part is made up of more than half by digits (1234aa@gmail.com)

OR

If there is at least one more digit than there are letters in the local part

OR

If there are at least 6 numbers in the local part

has spammy patterns:

If the email is longer than 20 and the top 3 most common characters make up more than 70% of characters

contains asd or sdf twice:

If the email contains the sequence of letters “asd” or “sdf” twice

local_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.

char repeated many times:

If any character is repeated at least 4 times

OR

If any pair of letters is repeated at least 4 times (tetetete@gmail.com)

domain contains short gibberish:

If the domain contains any of the following:

asdef

asdf

OR

If the domain looks exactly like any of the following:

asd.com

sdf.com

fsd.com

dsa.com

contains absurdity:

If the local part contains any of the following:

princessleia

If the local part is exactly:

sda

ads

dsa

nothing

abc

sdf

local no letters:

If the local part does not contain any letters

domain end is domain:

If the strings before and after the period in the domain are the same (logan@hello.hello)

domain contains phrase from list:

If the domain contains any phrase from the list of blacklisted phrases. Link to list.

local contains phrase from list:

If the local part contains any phrase from the list of blacklisted phrases. Link to list.

local low vowel ratio:

If the local part is 5 characters long and there are no vowels in it

OR

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)

Looks like spam: 

contains the phrase “noemail” anywhere in it

local part length is 1

the local part contains contains one character (a@gmail.com)

domain length is 1

domain contains exactly one character (logan@a.com)