Things to do when this happens:
Add any outside SMTP servers (example: spam filtering systems) and MX record to the Backup MX Hosts due to ratelimits.
Make sure Local Mail Exchanger is been set correctly.
Add MX IPs under the WHM >>> Exim Configuration Manager >>> “Sender verification bypass IP addresses” and “Trusted SMTP IP addresses”
IPs whitelisted under the server csf firewall.
Try disabling, and then enabling SMTP restrictions
WHM
Home »Security Center »SMTP Restrictions
Check the following
Check your email settings:
POP3: Incoming and Outgoing server: mail.[YOURDOMAIN].com
Username is your full email address, lowercase. Password is as provided to you. Outgoing server requires authentication. Do not use SSL unless you have your own SSL Certificate.
If it is only a sending problem, check the outgoing server port number. If it is 25, try setting it to 587.
