Q0411 - Exim/exim GitHub Wiki
Q0411
Why will Exim deliver a message locally to any username that is longer than 8 characters as long as the first 8 characters match one of the local usernames?
The problem is in your operating system. Exim just calls the
getpwnam() function to test a local part for being a local login name.
It does not presume to guess the maximum length of user name for the
underlying operating system. Many operating systems correctly reject
names that are longer than the maximum length; yours is apparently
deficient in this regard. To cope with such systems, Exim has an option
called max_user_name_length which you can set to the maximum allowed
length.