If you want to send as another user in Exchange 2013 or Exchange 2016 then we need to set permissions on the Exchange 2013/2016 server to do so. Once done we then need to setup the send from option in outlook. You can setup send as in Exchange 2013 and Exchange 2016 two ways, either send as or send on behalf. This can be done bia the EAC or power shell, first we will do it via power shell as it is very quick.
Exchange 2013/2016 Send On Behalf Powershell
To set this up simply drop into powershell and rin the following command.
Set-Mailbox UserMailbox -GrantSendOnBehalfTo UserWhowantstosend
Simply replace User Mailbox with the username of the user you want to send out from and User Who Wants To Send with the username who you want to give permissions to send on behalf to in Exchange 2013 and Exchange 2016.
Exchange 2013/2016 Send As Powershell
Again, to setup send as in Exchange 2013 and Exchange 2016 run the powershell below.
Add-ADPermission UserMailbox -ExtendedRights Send-As -user UserWhoSends
Again replacing usermailbox with the mailbox username you want to send from and User Who Sends with the username for who you would like to give rights to.
Exchange 2013/2016 Setup Send As and Send On Behalf In The EAC
To setup send as and send on behalf permissions in the EAC of Exchange 2013 and Exchange 2016 log in to the EAC and navigate to Recipients > Mailboxes then select the mailbox you would like to grant user permissions to send as or send on behalf. Once done click the pencil to edit. Then select which option you would like, either send as or send on behalf , click the plus sign and add the user who you would like to grant permissions to.
Setup Outlook to Send As
Outlook 2010 and Outlook 2013
In Outlook 2010 and 2013 simply click Options then tick From.You will now see the from box as seen here.