Send As
Classic Outlook
0. Start New Email
1. Select Options
2. Click Ellipses
3. Select From option under Show Fields
4. Select From box in email to change sender

Change Signature
0. Start Email
1. Change Sender in From field
2. Select Ellipses
3. Select Exclaimer
4. Change to your user
5. Select your Signature

- Video Demo -

New Outlook
Start New Email
Select Options
Select Show From option
Select From field below to change sender

Change Signature
0. Start Email
1. Change Sender in From field
2. Select Message
3. Select Exclaimer
4. Change to your user
5. Select your Signature

- Video Demo -

Adding Exclaimer Add-in
Go to:
aka.ms/olksideload

Wait for a few seconds
Search "Exclaimer" in search field

Select Add option

It will take a few minutes for the changes to go through and you may need to restart Outlook.