Exchange 2016 OWA and ActiveSync 503 error
Solution 1:
I have done some research, this issue may be related with certificate.
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Check from IIS to confirm which certificate that used for IIS service(Make sure the bind certificate is correct):
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Then run IISReset in CMD with administrator rights.
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Check if this issue still continues.
In addition, here's a similar thread for your reference: Exchange 2013 ECP/OWA/Outlook all failing - 503 Service Unavailable