Limits with sending emails from EC2 instance

Question

Hi,
We are providing  security awareness services to our customer. Which means we need to send phishing simulation and awareness training emails to hundreds (sometimes thousands) of users. we have installed a solution on ec2 instance. We are using built in mailserver (which is postfix) of the solution we use . we are not using port 25 ( we use 587) .We have tested and we can send emails to dozen of users. My question is; is there any limit with the number of concurrently sent emails? Like; if i send emails to 300 users, would some of those emails be blocked by aws?

thanks.

Kidd Ip · Answer

Email volume limits may depend on SMTP provider as well. If you're using Postfix to send directly to recipients (i.e., not relaying through a third-party SMTP), then:
• 	You may hit rate limits or spam filters from recipient mail servers.
• 	You’ll need to configure reverse DNS, SPF, DKIM, and DMARC records to improve deliverability.
• 	Some mail servers may reject bulk emails if your IP reputation is low or if you lack proper authentication headers.

Consideration:

- Throttle your sends: Use Postfix’s default_destination_rate_delay and default_destination_concurrency_limit to control how many emails go out per second.
- Use a relay SMTP service: Consider routing mail through a provider like Amazon SES, SendGrid, or Mailgun. These services are built for bulk email and handle reputation management.
- Monitor bounce rates: High bounce or spam complaints can get your IP blacklisted.
- Warm up your IP: If you're sending thousands of emails, start small and gradually increase volume to build trust with recipient servers.

Taz · Answer

Hey,

Hope you're keeping well.

AWS does not impose a hard limit on the number of emails you send from an EC2 instance over port 587, but deliverability will depend on your IP reputation and the receiving mail server’s anti-spam policies. If you’re sending directly from EC2, make sure you have proper DNS setup with reverse PTR records, SPF, DKIM, and DMARC to avoid being blocked. For large-scale campaigns, consider relaying through **Amazon SES**, which is designed for bulk email, offers high deliverability, and handles IP warm-up. You can also configure Postfix to throttle sends using `default_destination_rate_delay` and `default_destination_concurrency_limit` to avoid triggering rate limits on recipient systems.

Thanks and regards,  
Taz

Limits with sending emails from EC2 instance

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