电子邮件标题是否必须包含空格？

When I send a mail with php's build in mail() function and set a 'from' header it doesn't proparly work until it includes a space.

mail("sjorsvanholst@gmail.com", "Hello world", "This is a test", "From: I am") sends this mail (which is what I want):

with space

while mail("sjorsvanholst@gmail.com", "Hello world", "This is a test", "From: I") sends this mail:

enter image description here

Am I making a mistake here or this just how the function works?

EDIT: I should've included this but it isnt a charater length problem since 'Iamthisisalong' also just returns my orgin emailadres.

Other mail clients also have the same problem, I've tested it in Outlook, Outlook online and Gmail and all suffer from this problem