I originally wrote of a particular way to deal with this, but I’ve now found a better way and what I did is this:

  1. Go into Settings, Accounts & Passwords and select your Exchange account.
  2. Move the slider button next to Mail to the off position. This means this particular Exchange account is only allowed to manage the other services you need from Exchange: Contacts, Calendars, Reminders, Notes.

Now we’re going to set up separate account to handle Exchange email via iMap.

  1. Go into Settings, Accounts & Passwords and select Add Account.
  2. What you want to select is an account type of Other.
  3. Enter your name and the any Exchange email address you’re trying to set up (it doesn’t matter which because you can enter more later) under the IMAP Account Information section at the top. I gave it a description of ‘Exchange Email’.
  4. You then need to fill in the Incoming Mail Server and Outgoing Mail Server sections. The Username is your main Exchange account email and the Password is the one that goes with it. You then need a Host Name for each of these and it may vary depending on your Exchange set up but I use outlook.office365.com for the incoming mail server and smtp.office365.com for the outgoing mail server.
  5. Then hit the Done button. It’ll go away and check your account details and save the new mailbox.

Now to add additional email addresses:

  1. Go into Settings, Accounts & Passwords and select the account you set up above.
  2. Click on the email address next to Account.
  3. Click on the email address next to Email at the top in the _IMAP Account Information _section.
  4. Use the Add Another Email… button to add all your aliases. When you’ve entered a new email address, remember to press return afterwards otherwise it won’t remember it (you probably want the primary email for your Exchange account added here too).

You should find you can now click on the ‘From’ address when you start a new email and select the alias account as the sender.