the single id was not found in response Errors

Symptom

You may receive a POP3 connection error email stating that One or  more POP3 connections failed. 

It will display the following error in the details section:

Error: the single id was not found in response

Cause

When WHMCS attempted to import the message, it did not exist in the mailbox. 

This indicates that another program or email client was accessing the mailbox and removed the email at the same time as WHMCS.

Solution

When sharing a mailbox between devices and programs, we recommend using the IMAP protocol on each of them. 

To use IMAP for email importing in WHMCS:

  1. Go to Configuration > System Settings > Support Departments.
  2. Edit the affected support department.
  3. In the Mail Importing Configuration section, change the Port value to 993.
  4. Click Save Changes.