While checking the exim_mainlog for the failure issue:
20013-06-26 00:40:00 lowest numbered MX record points to local host: mydomain.com (while verifying email@example.com
To fixed the above problem:
This messages states that mydomain.com is using a remote Mail server and the entry for that domain exists in /etc/localdomains on local server. If mydomain.com doesn not use a remote Mail server, then entry for main domain should be in the file /etc/localdomains is missing and need to add it, manually.
Login to server using SSH edit such as vi or vim or nanao, add mydomain.com to the file:
Save and then restart Exim
# /scripts/restartsrv exim