By default, Microsoft Outlook always replies to and forwards mail using the account the message arrived on. Most of the time, this is how most of us want Outlook to handle our accounts. But there are times when you might want to send the reply from a different account.

If you always want to send all mail out using a specific email account (such as if you are changing addresses and the old address is inbound mail only), you can configure this in Account Settings by putting the sending email address in the Email address field (1), changing the outgoing server name (2) and configuring authentication for the outgoing server in More Settings, Outgoing Server tab (3). (The login username and password on the first page of the dialog is used to download mail.)

If you just want to choose a different address every now and again, for example, to forward a joke sent to your work account to friends using your personal account. To do this, you need to select the account in Outlook before sending, using the Account or From button.

If you need to frequently send mail from a different account, you can use a macro (below) to set the sending account, then assign the macro to a toolbar or ribbon button.

Macro to Reply, ReplyAll or Forward from a specific account

Outlook should always reply and forward from the account that downloaded the message. However, if you want to force replies or forwards from a specific account, you can use this code and create a custom reply button that uses your desired account, so you don't have to remember to change the sending account.

Replace the_email_account_name with the account name, as seen in the Account Settings dialog or on the Account selector.

To reply to all or forward, change
Set oMail = Application.ActiveExplorer.Selection(1).Reply

to either one of these:
Set oMail = Application.ActiveExplorer.Selection(1).ReplyAll
Set oMail = Application.ActiveExplorer.Selection(1).Forward

Always reply using a specific account

Public Sub New_Reply()
Dim oAccount As Outlook.Account
Dim oMail As Outlook.MailItem

For Each oAccount In Application.Session.Accounts

If oAccount = "the_email_account_name" Then
    Set oMail = Application.ActiveExplorer.Selection(1).Reply
      oMail.SendUsingAccount = oAccount
    oMail.Display
End If
Next

End Sub

To use a reply address you have SendAs rights to, you can use the .SentOnBehalfOfName property. Add it between the .SendUsingAccount and .Display lines (lines 9 and 10 in the sample above), as shown below. Don't forget to change the account name!

oMail.SendUsingAccount = oAccount
oMail.SentOnBehalfOfName = "me@here.com"
oMail.Display

To always send it to a specific address, add the recipient after the account is set:
oMail.SendUsingAccount = oAccount
oMail.Recipients.Add ("alias@domain.com")