Hi @Al Amran
Since OWA is working fine internally, I think the cause should be network problems.
Do you have some firewalls in the network blocking external requests to OWA?
Here are some links in the similar situation for your reference:
exchange owa error: error: Your request can't be completed right now. Please try again in a few moments.
OWA: Your request can’t be completed right now. Please try again later. problem
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