Issue with email - server edition

I’d been sending customers invoice via email until a few weeks ago it stopped working. There was an error message at the bottom says “Service Unavailable: Back-end server is at capacity” in red. Can someone point me in the right direction please?

You will have to offer more information. Are you using your own on-site mail server, what are your custom mail settings that last worked if any, does your normal email client use the same mail server or experience issues?

I did not setup custom SMTP server. I assume the server edition by default sends emails via’s email server. No, I have not change email settings.

In an attempt the solve the issue, I updated ubuntu server and rebooted the system. Now the error message looks a bit different “Error: NameResolutionFailure”

I’ve seen other users encountering the same problem but could not locate the thread. Any assistance is much appreciated.

Search the forum for “SMTP” you will find help or have a quick look at No luck with custom SMTP

Error: NameResolutionFailure means that you are still using an older version. Or that your server cannot resolve IP address of domain. Can you check what version are you on?

It may be worth your while to confirm DNS is functional on your Manager Host server. Carry out a nslookup from a terminal session.

Example (Not real IP data)

sudo nslookup


Non-authoritative answer:

The response you get from the Manager Server / host should come back with valid data. If not check your server’s IP config and DNS settings. You could also flush the DNS cache to trouble shoot further if the issue is related to DNS.

Backend capacity ? What does df -h give?