You are not using third-party tools on your PC, for example, Thunderbird
these applications save your email and you see the email offline too
Yes, I'm using Outlook but my clients can not access their accounts either and I can not speak for them when it comes to downloading an email.
Hopefully it will appear and help us with what is happening, when I try to log in to WHM it says "I can not read the license file"
"To access the interface, you must install the license and make sure the license is active."
This server does not have a valid license. To resolve this problem, you should contact your hosting provider.
Without answer, I am afraid that the data of my clients and mine may have disappeared.