Postfix → Dovecot LMTP: User does not exist: uid@domain

You need to set:

auth_username_format = %Ln

In conf.d/10-auth.conf

That removes the domain name, and the '@'.

The solution was to set

user_filter = (&(objectClass=posixAccount)(uid=%n)

in /etc/dovecot/dovecot-ldap.conf.ext.