You may receive this error in your email client settings at some point. In Outlook this is a bug and can usually be resolved by restarting your email client and making sure you have SMTP authentication enabled.
This is not a server problem and is caused by a miconfigured email client, ISP’s blocking access to port 25 and using port 26 instead or a DNS problem with your ISP.
To confirm that the mail server is online and responding you can perform the following:
From the ‘Start’ button click ‘Run’
Type in ‘telnet yourdomain.com 25’
You should receive something to the effect of:
220-server.hostname.com ESMTP Exim x.xx #1 day, xx Month 2020 time 220-We do not authorize the use of this system to transport unsolicited, 220 and/or bulk e-mail.
This confirms there are no problems with the SMTP server. To confirm the POP3 server is online and responding:
- From the ‘Start’ button click ‘Run’
- Type in ‘telnet yourdomain.com 110’
- You should receive something to the effect of:
+OK POP3 [cppop x.x] at [xx.xx.xx.xx] (yourdomain.com)
This confirms the POP server is online and there are no problems with the POP3 server.