The backend API service (PHP) deployed on Lightsail has been forwarding emails very slowly for the past few days.

Based on your description, the slow email forwarding issue on your PHP service in Lightsail could indeed be related to the Lightsail instance itself rather than your email server configuration (since you've already tried switching to Gmail).

This could be happening due to several Lightsail-specific factors:

1. CPU Credit Depletion: Lightsail instances use a burstable performance model. Even if your average CPU usage appears low, the email processing might be causing periodic spikes that deplete your CPU credits, causing performance to drop to baseline levels.

2. Resource Limitations: Your Lightsail instance might be running low on available resources (RAM, CPU) when processing emails, especially if you're on a smaller plan.

3. Background Processes: There might be background processes or updates consuming resources intermittently, causing delays in email processing.

To resolve this issue, consider these steps:

1. Monitor your instance: Use Lightsail's built-in monitoring tools to check CPU usage, burst capacity, and other metrics during both fast and slow email processing periods.

2. Check for resource bottlenecks: Review your instance metrics to identify if you're consistently hitting resource limits.

3. Optimize your PHP application: Review your code for inefficient email processing routines that might be consuming excessive resources.

4. Consider upgrading: If you're consistently hitting resource limits, you might benefit from upgrading to a larger Lightsail plan with higher baseline performance.

5. Reboot your instance: Sometimes a simple reboot can temporarily resolve performance issues.

6. Implement queuing: Consider implementing an email queue system to manage email processing more efficiently, especially during high-load periods.

If the problem persists after trying these solutions, you might want to consider migrating to a different service tier or AWS service that provides more consistent performance for your specific workload.
Sources
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Hi,

Please following the guide her to configure reverse DNS for your email server: https://docs.aws.amazon.com/en_us/lightsail/latest/userguide/amazon-lightsail-configuring-reverse-dns.html

For more information about email server setup in Lightsail, refer to: https://docs.aws.amazon.com/lightsail/latest/userguide/amazon-lightsail-enabling-email-on-wordpress.html

Best, Joy