As of this time now, the Fedora Maintainer did not react to the bugreport or started a new build, so you get the quick & dirty fix from me now:

Fedora: Exim 4.99-1 packages have defective mysql backend

All Exim 4.99-1 builds for all architectures have the same mysql backend bug: it does not work anymore.

This is caused by a configure mistake the maintainer has done before compiling the source code. As a result, the refered to mysql-lookuplibraryname is wrong:

traditionally, Fedora Exim used this name schema: „/usr/lib64/exim/<version>/lookups/mysql.so“, but the Exim 4.99-1 build referes to :

openat(AT_FDCWD, "/usr/lib64/exim/4.99-1.fc42/lookups/mysql_lookup.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)

failing to find, what is not existing there.

The Workaround

The quick workaround is to symlink the used name to the real lib name:

cd /usr/lib64/exim/4.99-1.fc42/lookups
ln -s mysql.so mysql_lookup.so

You do not even need to restart Exim, because this is not a runtime issue.