Use this code to send a new message using the address the selected message was sent To as the send using From account.

See Using a Gmail Master Account: Reply using the Correct Account for instructions and an automated set of macros. It will work with any service that collects mail sent to other accounts.

This code is for people who use an account such as Gmail or Outlook.com to collect mail from other accounts and download only that account. Outlook will always reply using the account that downloaded the message, which is the Outlook.com or Gmail account. You'll need to create an account in Outlook for the other addresses to use this code, but can set the accounts to never download email.

This code gets the address the message was sent to and looks for an existing account with the same display name and uses that account. If it can't find a matching account it uses the account that downloaded the message. This could be changed to use the default account in your profile.

Update: I fixed the macro, it now checks for a specific string (an email address) in the To field. If it finds the address, the reply is from that address, otherwise the reply is from the default address. If you need to check for one of several accounts, you'll need to use an array.

To force the code to always use a specific non-default account, change this line:
objMsg.SendUsingAccount = olNS.Accounts.Item(1) to
objMsg.SendUsingAccount = olNS.Accounts.Item(2) where 2 is the placement of the account in the Account Settings list
or objMsg.SendUsingAccount = "Display name of desired account"

If you are using Exchange server and have Send as permission to the address a message was sent to (such as a distribution list), replace the For Each... Next block with:
objMsg.MailItem.Sender = oMail.To
or
objMsg.SentOnBehalfOfName = oMail.To

This may not work with Outlook 2013's "inline replies". Messages will be opened in a new Window.

Reply from the address a message was sent to

Public Sub AccountSelection()
Dim oAccount As Outlook.Account
Dim strAccount As String
Dim olNS As Outlook.NameSpace
Dim objMsg, oMail As MailItem
 
Set olNS = Application.GetNamespace("MAPI")
 
' For a reply all version, replace Reply with ReplyAll 
Set objMsg = ActiveExplorer.Selection.Item(1).Reply
 
If TypeName(ActiveExplorer.Selection.Item(1)) = "MailItem" Then
 Set oMail = ActiveExplorer.Selection.Item(1)
  
 On Error Resume Next
 
For Each Recipient In oMail.Recipients
 strRecip = Recipient.Address & ";" & strRecip
Next Recipient
 
 
If InStr(strRecip, "alias@domain1.com") = 1 Then
' StrAccount is the account name as shown in the Account Settings list
strAccount = "alias@domain1.com"
Else
End If
  
 For Each oAccount In Application.Session.Accounts
  If oAccount.DisplayName = strAccount Then
     objMsg.SendUsingAccount = oAccount
       Else
 
     ' to reply using the account that downloaded the message
     ' leave the objMsg line commented
     ' remove comment to reply using default account
     ' objMsg.SendUsingAccount = olNS.Accounts.Item(1)
 
  End If
Next
 
  objMsg.Display
 
Else
 
End If
 
Set objMsg = Nothing
Set olNS = Nothing
End Sub

Check for multiple aliases

This version of the macro, updated by Victor Beekman, checks for multiple aliases.

Public Sub AccountSelection()
Dim oAccount As Outlook.Account
Dim strAccount As String
Const strAlias1 = "alias1@somedomain.xxx"
Const strAlias2 = "alias2@somedomain.xxx"
Dim strRecip As String
Dim olNS As Outlook.NameSpace
Dim objMsg, oMail As MailItem
Dim Recips As Object
Dim i As Long

Set olNS = Application.GetNamespace("MAPI")
' to integrate with replyall button see https://www.slipstick.com/developer/remove-address-reply/
' For a reply all version, replace Reply with ReplyAll
Set objMsg = ActiveExplorer.Selection.Item(1).ReplyAll
 
 On Error Resume Next
 
Set Recips = objMsg.Recipients
For i = Recips.Count To 1 Step -1
  If LCase(Recips.Item(i).Address) = strAlias1 Then
     strAccount = strAlias1
     Recips.Remove i
  ElseIf LCase(Recips.Item(i).Address) = strAlias2 Then
     strAccount = strAlias2
     Recips.Remove i
  End If
Next i
  
 
If InStr(strRecip, strAlias1) = 1 Then
' StrAccount is the account name as shown in the Account Settings list
    strAccount = strAlias1
ElseIf InStr(strRecip, strAlias2) = 1 Then
    strAccount = strAlias2
End If
  
For Each oAccount In Application.Session.Accounts
  If oAccount.DisplayName = strAccount Then
     objMsg.SendUsingAccount = oAccount
       Else
      ' to reply using the account that downloaded the message
     ' leave the objMsg line commented
     ' remove comment to reply using default account
     ' objMsg.SendUsingAccount = olNS.Accounts.Item(1)
   End If
Next
objMsg.Display

Set objMsg = Nothing
Set olNS = Nothing
End Sub

How to use macros

First: You will need macro security set to low during testing.

To check your macro security in Outlook 2010 or 2013, go to File, Options, Trust Center and open Trust Center Settings, and change the Macro Settings. In Outlook 2007 and older, it’s at Tools, Macro Security.

After you test the macro and see that it works, you can either leave macro security set to low or sign the macro.

Open the VBA Editor by pressing Alt+F11 on your keyboard.

To put the code in a module:

1. Right click on Project1 and choose Insert > Module
2. Copy and paste the macro into the new module.
3. Click Run to test the macro

When you're happy with the macro, create a ribbon or QAT button for the macro.

More information as well as screenshots are at How to use the VBA Editor