Sign into webmail and if same issue is occurring there go to ☰ > Settings > ▸ Email, scroll to bottom of page and select Change IMAP subscriptions and make sure all folders are selected. If the changes are not reflected in webmail's folder view press Ctrl + Shift + R.
Alternatively check IMAP subscription from within Outlook.
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It would be helpful to see what, if any, VM error code is returned by the SMTP server in this situation. Open a Terminalwindow and enter the following command where username:password is replaced with your email address and password, firstname.lastname@example.org:Y54kpX5g: