I'm having the same problem. Some service providers seem to be blocking our server now due to excessive retries.
Here is one that has over 5,000 attempts. Others are at over 20,000. It seems like in many of these cases, one of the recipients has a permanent error. In the email below, one of the recipients does not exist on the email server, yet our server continues to retry it.

[2020.09.23] 14:10:37.826 [72489957] Attempt to ip, '198.58.118.204' success: 'True'
[2020.09.23] 14:10:37.826 [72489957] Delivery for theduck@mcgonigels.com to orders@mcgonigels.com has bounced. Reason: Remote host said: 550 No Such User Here"
[2020.09.23] 14:10:37.826 [72489957] Removed from RemoteDeliveryQueue (1 queued or processing)
[2020.09.23] 14:10:39.060 [72489957] Added to RemoteDeliveryQueue (1 queued; 5/20 processing)
Written
theduck@mcgonigels.com
orders@mcgonigels.com
retry: 5604;09/21/2020 22:25:04
from: theduck@mcgonigels.com
auth: [snip]
forwardingDomain: tecture.com
creationdate: 09/21/2020 14:06:17
priority: Low
priorityDetermined: True
dsnreturntype: hdrs
notify: theduck@mcgonigels.com=
notify: [snip]=
notify: [snip]=
containsLocalDeliveries: True
unfoldedRecipient: admin@tecture.com
unfoldedRecipient: [snip]
unfoldedRecipient: [snip]
connectedip: 3.18.57.153
helo: mcgonigels.com
connectedhostname: ec2-3-18-57-153.us-east-2.compute.amazonaws.com
smtpSessionId: 31590496
Return-Path: <theduck@mcgonigels.com>
Received: from mcgonigels.com (ec2-3-18-57-153.us-east-2.compute.amazonaws.com [3.18.57.153]) by hyperion.tecture.com with SMTP
(version=TLS\Tls12
cipher=Aes256 bits=256);
Mon, 21 Sep 2020 14:06:17 +0000
Date: Mon, 21 Sep 2020 14:06:16 +0000
To: theduck@mcgonigels.com, orders@mcgonigels.com
From: "[Rusty]" <theduck@mcgonigels.com>
Reply-To: [snip]@gmail.com
Subject: McGonigels Contact Form Message [snip]
Message-ID: <ff63c342249b618c9df6d4d422660434@mcgonigels.com>
X-Mailer: WPMailSMTP/Mailer/smtp 1.8.1
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8