Why does base64 get corrupted inside an email?

Question

linux

I can't figure out why an email with a base64 string in img is sent correctly from one server, but not from another server.
Here is an example of the difference, a snippet of a line
from a working example:
Izj8KyUbSSYQ8zFtbK5c5e1dZtwby25WRe4yPx4Oc5qy9y91mEh
in an email where base64 got corrupted:
Izj8KyUbSSYQ8zFtbK5c5e1dZtwby25WRe4yPx4Oc5qy!
9y91mEh
Note that the character "!"
Why is this happening? Input data (i.e. the string itself that is generated by base64_encode in PHP is correct)

Answer 1

And the letter is initially correctly generated?
RFC 2045

6.8. Base64 Content-Transfer-Encoding
The encoded output stream must be represented in lines of no more than 76 characters each.