Unable to acknowledge alarm by replying to email

We have been using an Email Alarm Notification Profile for a while to send alarms to all of our operators, once they receive the email they can reply to the email to acknowledge the alarm, and we enabled the POP3 two-way for acknowledging alarms. We have noticed that certain operators are not able to acknowledge alarms, but other operators can. I have confirmed that we successfully received the reply email, but the alarm is not acknowledged in Ignition gateway and the email stays in the email inbox. While other operators’ replies would work and the emails get deleted from the inbox because I have selected “Delete on Acknowledge”.
What are some of the things I can troubleshoot? Our users list comes from Active Directory, I verified that the name of the operators and email addresses are correct. This used to work fine, we recently changed the email addresses of all the operators and this issue started to come up, but it only happens to a select few staff. We are running version 7.9.5.