Exchange 2010 Hybrid Configuration Wizard Web Services External Url

DISCLAIMER: While the below "should" work...I'd be cautious about undertaking it as your "backup" solution, as that's not what it is designed for.

Because you haven't setup O365 at all, in theory you could do the following, assuming you plan on going to O365 within 60 days (I believe that's the limit):

• Setup O365 for a cutover migration (you need to really understand what a cutover migration is, how it works, etc. before proceeding) but don't actually DO the cutover. Just the staging part. You'll setup the TXT record for the domain, and then setup the Exchange migration sync part.
• Start the sync and just let it sync each day for each mailbox
• DON'T, repeat DON'T finalize the cutover migration in the portal until you are fully ready to cutover...as once you do syncing will stop.

Note: this only handles mailboxes, not public folders or anything like that.

Basically what you'll be doing is a cutover migration but you'll be taking your time doing it. If all mailboxes sync within a few days, then at that point if your Exchange server on AWS fails, you could basically force the finalizing of the cutover quickly (MX records at this point, finish the cutover wizard, etc.) and retain all the email up to the last sync (happens every 24 hours or manually).

That's about the best I can think of.

A trusted SSL certificate from a trusted 3rd party like Verisign or similar is required to do a cutover migration to Office 365.

This is why you are getting the errors.

You'll have to purchase and install a legitimate trusted certificate from a trusted online CA in order to proceed.

You could probably get by with Comodo's free 90 day cert: https://www.comodo.com/e-commerce/ssl-certificates/free-ssl-certificate.php but I've never run into a situation where we weren't using a true trusted certificate.