Please branch and build mailx in epel9. If you do not wish to maintain mailx in epel9, or do not think you will be able to do this in a timely manner, the EPEL Packagers SIG would be happy to be a co-maintainer of the package; please add the epel-packagers-sig group through https://src.fedoraproject.org/rpms/mailx/addgroup and grant it commit access, or collaborator access on epel* branches.
Hello Davide, do you really need mailx? In RHEL/CentOS 9 there is s-nail, that is compatible with mailx (it even provides mailx command) and has many more features.
We can get by with s-nail, thanks for checking.