apply Mailbox Rights permissions for multiple users at once

In preparation for a migration to Office 365 Exchange Online, I need to ensure Outlook Web Access is working for all users on the current on-prem SBS 2003 Exchange configuration.
It wasn't configured at all and I've since run the Internet connection and Email wizard to enable Outlook Web Access and for a newly created user in the SBS Users in AD, is working fine.  The user can log in without issue.
But for all existing users, it just keeps prompting for the username and password.
I've found that it is because the users do not have the "Associated external account" set to Allow for the user "SELF" in Mailbox Rights for these accounts.
I can enable it, but considering there's over 30 users, I was hoping there was a quick way to change it for all users.

Is there any way this can be done?
Reece DoddsAsked:
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For 30 I'd do that manually. I'd start searching for something automatic from 300 o 3000 mailboxes.

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Reece DoddsAuthor Commented:
looked at using ADModify to export the mailbox rights for the selected users, modifiying the XML and then importing it, but it didn't work.
Manually it is.
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