Exchange 2010 ssl certifiate with multiple domains for Hybrid setup

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We have Exchange 2010 SP3 with 200 users with Mimecast as Email security with different SMTP domains like,,

All are in single AD domain "".

We would like to migrate one SMTP domain ( users' (around 50) mailboxes onto Exchange online. Planning to change their UPN from AD domain to SMTP domain ( to for AAD sync.

On exchange server we have wildcard certificate for configured for SMTP and IIS. 

Do we need to new public SSL certificate for for OWA/ActiveSync/autodiscover for Office 365 migration as HCW looks for certificate?

If generate CSR from Exchange server, it is adding all three domains in the list. Shall keep all entries as Exchange added or keep only the required domain and remove other?

Do i need to assign new certificate to SMTP/POP/IIS? Will it overwrite existing ( certificate?

It's confusing for me how to handle certificate for Hybrid migration for one SMTP domain from Exchange 2010 server hosting multiple SMTP domains.

Thanks in advance for your help.

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