Improving public outbound on my SMTP server.

I have my configured my SMTP server for mass emailing. My mail client was able to send approximately 100 emails per seconds and was using 40 connection at the same time.

On my droplet usage, it shows that there was a peak in inbound which averages 2 mbps but my outbound was only 0.5 mbps.

What does that mean? Does it mean that not all emails has been sent? Does it mean that the relay is much slower compare to what was received my the server which means, all emails will be sent except that it will take more time to get relayed?

How to improve outbound performance? More CPUs?

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Without knowing which MTA you’re using, then it’s difficult to know. And your logs should give you an idea on what’s going on. By default DigitalOcean blocks (and sometimes limits) new accounts ability to send mail to prevent spam. Create a ticket with Support in the control panel to make sure you’re not being throttled.