You may see the error “An Error occurred SMTP Error: could not authenticate” in your whmcs while sending a mail or an invoice.
The issue could be caused by both firewall on the server and the settings at your WHMCS. Please follow the steps to resolve the issue.
1) Login to the server via SSH.
2) Open CSF and make sure ports 25,465 and 587 are in the allow list.
3) Make sure SMTP_port is set to 25.
4) Save and exit then restart the firewall.
Now at the WHMCS end do the following:
5) Login to your WHMCS.
6) Go to General Settings.
7) Open the mail tab and change the SMTP port number to 25.
8) Also make sure that the SMTP host is mentioned as ‘localhost‘.
9) Save the changes.
The issue should be resolved now, if it still persists, you may check whether the mail authentication details mentioned in the WHMCS has changed or not. If you need any assistance, please contact our support department.
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