This comes up a lot these days, note that the "Send As" changes in EXO, also do not get confused with the differences, shown below:
Send on Behalf as : This is usually when people use Outlook to add a "from" field, as you are sending on behalf of your mailbox.
Send As : This is when you connect to the mailbox directly and "sendas" that mailbox for example if Junior Bear and doing a SendAs from Grizzly .Bear - usually you would visit the "another mailbox" URL like this > https://outlook.office.com/Grizzly.Bear@bears.local
These are the commands you need:
Send on Behalf
You need to get the correct command else you will override the current entries and remove people already added, remember this command will override not append!
First User to be Added:
Set-Mailbox "Grizzly.Bear@croucher.cloud" -GrantSendOnBehalfTo "lee@croucher.cloud"
If you wish to "add" another user:
Set-Mailbox "Grizzly.Bear@croucher.cloud" -GrantSendOnBehalfTo @{Add="new.user@croucher.cloud}
If you wish to remove a "single" user:
Set-Mailbox "Grizzly.Bear@croucher.cloud" -GrantSendOnBehalfTo @{Remove="new.user@croucher.cloud"}
Send As
This will work the same way as the "Send on Behalf" of for permissions, be careful with these commands:
First User to be Added:
Add-RecipientPermission -Identity "Grizzly.Bear@croucher.cloud" -AccessRights SendAs -Trustee "user@example.com" -Confirm:$false
If you wish to "add" another user:
Set-RecipientPermission "Grizzly.Bear@croucher.cloud" -AccessRights SendAs -Trustee @{Add="anotheruser@example.com"}
If you wish to remove a "single" user:
Set-RecipientPermission "Grizzly.Bear@croucher.cloud" -AccessRights SendAs -Trustee @{Add="newuser@example.com"; Remove="olduser@example.com"}
Full Mailbox Access
This will grant Junior.Bear full access to Grizzly.Bear's mailbox.
Add-MailboxPermission -Identity Grizzly.Bear -User Junior.Bear -AccessType FullAccess -AutoMapping $False