Add Position Email Account and Send As permission within "Outlook"

You need to check (RBAC - Role Based Access Control)

Organization Management
Recipient Management
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Power Shell Command
Take Full Control on "IT Support" user's mailbox

Add-MailboxPermission -Identity "IT Support" -User "Zaw Htet" -AccessRights FullAccess -InheritanceType All

Give Send As permission

Add-ADPermission -Identity "IT Support" -User "Zaw Htet" -AccessRights ExtendedRight -ExtendedRights "Send As"    
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Set-Mailbox itsupport -type:shared

1. You will need to find that user in AD and go to the properties >> Security >> Advanced:
2. You need to ENABLE the option to "Include inheritable permissions from this object's parent":
3.

Add-ADPermission itsupport -User mmx\zawhtet -ExtendedRights "Send As"

Check Send As

Get-Mailbox | Get-ADPermission | where {($_.ExtendedRights -like "*Send-As*")} | FT -Wrap 

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Clear Submission Queue

Stop-Service MSExchangeTransport

C:\Program Files\Microsoft\Exchange Server\Transport Roles\Data\

Here you will find a folder named Queue .Rename this folder give it a name like Queue2.Create a new empty folder named Queue.

Start-Service MSExchangeTransport

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get-exchangeserver | test-servicehealth