Hello, @Said A,
Welcome to the Microsoft Q&A platform!
The default timezone for shared mailboxes seems to follow the current windows timezone.
Here is what I have practiced to check the shared mailbox timezone and found it to be empty if not set:
Get-MailboxRegionalConfiguration -Identity “<Shared_mailbox_name>”
Then I sent emails to my personal mailbox as well as the shared mailbox using the shared mailbox, then logged in to the shared mailbox from outlook and OWA respectively, and the same email viewed in the shared mailbox using OWA login does not match the timezone set, even though OWA has other timezones set.
The first is an OWA mailbox with a time zone set. The second and three are logging into the shared mailbox and windows time via that mailbox (screenshot delayed by a few minutes) For the factors you mention as possible influences, they seem to only affect personal mailboxes (outlook on the left, OWA on the right)
Should you need more help on this, please feel free to post back.
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