Outlook account disappeared from settings and From-field, but not from sidebar tree?

I am a webmaster of a website that has a few email addresses under its domain. These email accounts could be configured as both IMAP and POP3.

For one of the users, a strange issue happens, and seemingly has happened before as well.

Their email account is still receiving emails, but they can't seem to choose the email address for sending emails. I first thought this might be caused by wrong settings, except that they actually are unable to choose the specific address in the "From:" dropdown-field when composing an email.

Aditionally, when I asked them to update their settings to make sure they had the accounts configured correctly, they replied by saying they couldn't update the settings anymore, because it is not present in the settings and account information dropdown box. Here I posted an image to show you all the places in which Outlook seemingly "erased" the account

As I mentioned before, they still receive emails for these accounts, and the account is still accessible in the sidebar tree from Outlook. Here I posted an image to show you what I mean with the sidebar tree from Outlook

Just to summarize:

• They don't see the email account in the "From:" dropdown field when composing an email.
• They don't see the email account in the account settings list.
• They don't see the email account in the acount information dropdown list.
• However, they do see the email account in the sidebar tree, and they are still able to receive emails.

So, I was wondering what is going on here, and how can it be solved? They do seem to have had this problem before, so I am wondering if it is an exteal factor that should be removed to prevent it from happening again.

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