If you are using cPanel server and you might get this error from your exim_mainlog very often whenever sending and receiving email.
temporarily rejected RCPT <email@example.com>: lowest numbered MX record points to localhost
The reason of this error message is the domain.com is missing from either the /etc/localdomains file or /etc/remotedomains file. The difference between two of these file is that, they decide whether the email has to be delivered locally or through a remote server by the exim email services.
To get it fix, add your domain.com to /etc/localdomains file to last line(new line each domain).