We are no longer receiving notifications from our contact form

It sounds like you're experiencing an SMTP (Simple Mail Transfer Protocol) issue with your WordPress contact form notifications. This is a common problem that can occur for several reasons when using AWS services. Here are some potential causes and solutions:

1. EC2 Port 25 Throttle: If your website is hosted on an EC2 instance, AWS throttles outbound traffic on port 25 by default, which can prevent sending emails directly via SMTP. You have two options:

• Apply to AWS to have the port 25 throttle removed for your account
• Use AWS SES SMTP interface on port 587 or 2587 instead

2. AWS SES Configuration: If you're using AWS Simple Email Service (SES):

• Check if your SES account is in sandbox mode, which limits sending emails only to verified addresses
• Verify your SES credentials are correct in your WordPress configuration
• Check if your SES account might be under review (this would affect email delivery)

3. WordPress Plugin Solution: Consider using an SMTP plugin to help resolve email delivery issues:

• WP Mail SMTP
• Easy WP SMTP
• Post SMTP Mailer/Email Log

These plugins can help you configure and test your SMTP settings and often provide logs to troubleshoot issues.

4. Email Verification: If you recently added a new email address for notifications, check if it requires verification. AWS often sends verification requests that must be confirmed before notifications can be delivered.

Since you mentioned this is critical for your business, I'd recommend implementing an SMTP plugin as a quick solution to help diagnose the issue and potentially fix it by providing better error logging and configuration options.
Sources
SMTP Issues on Website | AWS re:Post
The Amazon SES Account is under revision process but there's nothing on the AWS Personal Health Dashboard | AWS re:Post
Troubleshoot notification delivery in Lightsail - Amazon Lightsail