I have a VPS server with CPanel on my domain (say mydomain.com). I have a SuiteCRM installed on a subdomain (say sub.mydomain.com) and I am sending mass emails from it in a batch of 90 every hour.
While sending the emails from the CRM (Authenticated SMTP mode) the emails are getting deferred with the error:
SMTP error from remote mail server after MAIL FROM:< myemail@mydomain.com> SIZE=562346: 451 <myemail@mydomain.com> server temporarily unavailable. AUP#MXRT
The SPF, DKIM and DMARC everything is fine.
I am suspecting it could be a max rate warning/failure (MaXRaTe hence MXRT) from the remote/recepient mail server. It is failing/deferring emails for Gmail, Yahoo and other email providers not for any company-specific emails like someone@somecompany.com
There seems a limit on emails for Gmail and other vendors. I had gone through these where Gmail specifically pointing out the limits and do's and don'ts:
https://support.google.com/mail/answer/81126
In my case, I find myself well under the limit. So, why I am getting this error?
Second thought: It could be due to that I am sending 90 emails in one go. If I send emails one-by-one after a delay (say 3-5 seconds) then there won't be any problem. Enlighten me on this too.
I later realized that, even any one-to-one email I am sending via Thunderbird from my server (SMTP), even to a single recepients, is failing. I have checked check.spamhaus.org and intodns.com everything is alright there.
My Environment (Though this hardly matter in context of my problem but just in case):
BigRock's VPS Server with CPanel (Version)
Apache Version: 2.4.53
MySQL Version: 8.0.29
WHM Version: 102.0.18
CentOS Version: 7.9.2009 (Standard KVM)
Note: I haven't mentioned my actual domain name just for the privacy and security since I feel putting everything here will pose risks.
