Sending multiple emails from my smtp-relay avoiding DoS

We are sending emails each 4 hours 3 times per working day. 

After a certain quantity of sent emails (near 2000 every day, never an exact number), we are getting the following return starting with:

Code:

220 smtp-relay.gmail.com ESMTP u13-XXXXXXXXXXXXXX023asm946523vkl.5 - gsmtp
<br /><pre>hello: 421 4.7.0 Try again later, closing connection. (EHLO) .............

Studying the case, I've reached at https://support.google.com/a/answer/2956...ay-service :



Taking a look in the email library in CI3 user guide in Email Preferences ( https://codeigniter.com/userguide3/libra...references ), I've found smtp_keepalive.


Does someone know if the goal of smtp_keepalive would solve the "connection caching" mentioned in Google documentation above? Otherwise, what would be the solution to configure this "connection caching" when sending emails using the email library in CI3?

Thank you!