WordPress Newsletter Deliverability Setup

I run my bulk mailings through the “Newsletter” plugin on WordPress and need an expert to tighten up deliverability and, most urgently, get reliable bounce processing in place. Right now there is some configuration error causing bounce loops and triggering Google Workspace rate limiting.

Initial Problem – September 2026
On 1 September 2026, during/after a normal newsletter campaign, Google Workspace suddenly began reporting recipient rate limiting against the dedicated bounce mailbox.
Google Workspace generated the following alert:
Recipient: bounce@.....org.za
Messages rejected: 2,269
Started: 2026/09/01 13:10 UTC
Last rejection: 2026/09/01 14:06 UTC

Google reported:
The user was having mail delivery rate limited due to an abnormally high rate of delivery. Google indicated that this could be caused by a denial-of-service condition or a mail configuration resulting in mail loops.
We investigated the event using Google Workspace Email Log Search.
The high-volume traffic consisted primarily of failed delivery/bounce notifications generated as a result of the newsletter campaign.
This behaviour had not previously caused Google Workspace rate-limiting alerts despite the newsletter system having been used for many years.

REQUIREMENT
- Reliable delivery of newsletter campaigns.
- Correct config for handling of hard/soft bounces
- No high-volume NDR/bounce traffic entering the normal info@ Google Workspace mailbox.
- No Google Workspace recipient rate-limiting caused by newsletter activity.
- Correct SPF, DKIM and DMARC alignment.
- No mail-routing loops.
- Continued normal operation of existing Google Workspace users.
- Appropriate suppression/management of invalid newsletter subscribers.
- A configuration suitable for campaigns to a mailing database of approximately 149,000 subscribers.
- Where configuration changes are required, we would like them documented so that the resulting mail flow and responsibilities of Google Workspace, server03/Exim and the WordPress Newsletter - - Plugin are clearly understood.

Systems Involved
- WordPress
- The Newsletter Plugin for WordPress
- WP Mail SMTP
- cPanel / WHM
- Exim
- Google Workspace / Gmail
- Google Workspace Email Log Search
- DNS (MX/SPF/DKIM/DMARC)
- server03.santashoebox.org.za
- Domain: santashoebox.org.za