1. Check first the conneciton:
1.1. Open below URL from your computer:
1.2. Ensure you're on the "Exchange Server" tab.
1.3. Select "Service Account Access (Developers)" under the "Microsoft Exchange Web Services Connectivity Tests" heading.
1.4. Click Next
1.5. For the "Target mailbox" enter the end-user's mailbox you are trying
1.6. For the "Service Account User Name" enter the admin account used
to access the target mailbox.
1.7. Select "Use Autodiscover to detect server settings". (It should be selected by default)
1.8. Check "Use Exchange Impersonation" and enter the end-user's mailbox you are trying
1.9. Leave "Ignore Trust for SSL" disabled.
1.10. Check below option:
"I understand that I must use the credentials of a working account from my Exchange domain
to be able to test connectivity to it remotely. I also acknowledge that I am responsible
for the management and security of this account."
1.11. Click Perform Test.
2. ODME could be also unable to to resolve target mailbox's settings via autodiscover.
Use then "<targetdommain>.onmicrosoft.com" email addresses as target mailbox names.
It will allow ODME to use autodiscover for getting target mailboxes settings successfully
as autodiscover for "<targetdommain>.onmicrosoft.com" should always work