Exchange Online user with Office 365 cannot send mail as only on behalf

We are new to Exchange Online/Office 365 and I have a user who used to be able to send email as a secondary mailbox they had access to in their Outlook and now it shows up as send on behalf.  How can this be set back?

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You can try these commands for the Windows Azure Powershell
$LiveCred = Get-Credential

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri -Credential $LiveCred -Authentication Basic -AllowRedirection

Import-PSSession $Session

To verify that the permissions send mail on behalf of another user:
Get-Mailbox -identity | fl *GrantSendOnBehalfTo

To remove Send On Behalf permission from a mailbox, use the following command:
Set-Mailbox -Identity -GrantSendOnBehalfTo $NULL
Vasil Michev (MVP)Commented:
As comfortjeanius suggested, you need to verify the permissions. It's easiest to do this using the EAC:
bergquistcompanyAuthor Commented:
Ok user has send as and full permisson to the mailbox
She has it added under her mailbox and she can reply and change the from to the secondary mailbox  but when in it doesn't default to secondary.
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Vasil Michev (MVP)Commented:
Well the default one is configured by setting the default account in Account settings, it will not change based on the context. If this is a normal mailbox, she can add it as separate account in Outlook and it will auto-update the send address depending on which account she is working with (on Outlook 2013 at least).

If this is a shared mailbox however, the send from address will not change to it. As a workaround you can add the shared one as POP account so you can set it as the default, but again it's not an ideal solution.
To assign "Send on behalf of " permissions
Set-Mailbox -Identity <DisplayName A>  -GrantSendOnBehalfTo <DisplayName B>

Open in new window

To remove "Send on behalf of" permissions
Set-Mailbox -Identity <DisplayName A> -GrantSendOnBehalfTo $null

Open in new window

Plus here is some information for your safe keeping Mailbox Permissions - PowerShell commands
bergquistcompanyAuthor Commented:
@ comfortjeanius - she does not want to send on behalf, but send as

@ vasilcho - ok to clarify If I have my mailbox open in Outlook and a secondary with full access and send as.  If I am in the secondary mailbox under my account and reply it will NEVER send as that user as it is under my account?

I want to make sure I'm clear as I can just change the from field and it will work, but the user would like to avoid that step and told me it used to work so want to make sure I am clear it is not possible.
Vasil Michev (MVP)Commented:
I did some digging after, seems there is a known issue with sending from shared mailboxes in O365 (wave 15):

Our users have also confirmed that it worked before, so I was wrong in my previous post.

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bergquistcompanyAuthor Commented:
You nailed it thanks!
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