Outlook - Need to change Reply from address for a shared mailbox.

Issue:

We have a shared mailbox called “clientassist.”  We can send emails as clientassist if we choose the account from the “From” dropdown:

But we have to it every time. Replies and new emails always default to the primary account holder’s email address.

Is it possible to have Outlook automatically choose the clientassist shared mailbox account if the user is in the clientassist “Inbox?”

Explanation:

Whenever two accounts are added in Outlook the "From" address field populates as per below:

When composing new emails:

The account selected as Default will appear in "From" field whenever we compose new emails.

Above mentioned behavior will only work if this option is selected, " Always use default account when composing new messages" 

To enable this option:

Go to File > Options > Mail > Send messages > Always use default account when composing new messages (Mark the option)

When replying to emails:

From address field populates as per the mailbox in which the email was received.

Example: 

There are two mailboxes [e-mail] & [e-mail], default address is [e-mail].

The email was received on [e-mail] mailbox, when replying back to this email the From address field will be [e-mail] and vice-versa.

Solution:

In order for the Shared Mailbox to show as the default FROM address when replying to emails within it's mailbox you have to follow the following steps:

1) Make sure that the user has full access but has auto-map to Outlook client disabled, this can only be done through PowerShell; to assign permission follow this step:

• First go to the Exchange Admin Center then navigate to the Shared Mailbox and Remove all users with Full Access Permission that want to have the option to have default reply address of shared mailbox. (make sure to take a note of users if there's a long list)
• Open PowerShell and run it as admin
• Run commands:

Set-ExecutionPolicy RemoteSigned

$UserCredential = Get-Credential

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection

Import-PSSession $Session -DisableNameChecking

Add-MailboxPermission -Identity [e-mail] -User [e-mail] -AccessRights FullAccess -InheritanceType All -AutoMapping $false

2) Restart the user's Outlook and the Shared Mailbox should no longer be showing or auto-mapped. It could take 10-15 minutes before it disappear.

3) Now add the Shared Mailbox as a separate account so it will have it's own data file and have its address as the default when replying to email within it's inbox:

• Open Outlook
• Go to File > Add Account
• Type in the email address of the shared mailbox
• When prompted to provide password, click on the option that says Sign in with another account.
• Now, use the user's credential that has full access and you should be able to connect the shared mailbox as another account instead as an auto-mapped mailbox.

4) Test by going to inbox of the shared mailbox then select an email and click on reply.