I've noticed a few problems with Emails not being sent to Exchange addresses via php mail. First I noticed this when using FreeForm. I didn't notice anything out of the ordinary while testing to my gmail account. Forms was successful and I received the emails.
But then the customer said they hadn't received any emails. So I checked it out and added my own Gmail address to the recipients. And I did receive the emails while testing. But the customer did not. Even though both our addresses were set as recipients.
So I narrowed it down to the customer's email being an Exchange address. As a temporary solution, I added the customer's gmail address as recipient. And that works. And since Freeform stores each submitted form on it's own, I had a record of every form.
But now we have the same problem on a different site with Expression Engine's own Form. And not freeform. And we have no record of the forms that's been sent. And the customer is not very happy.
Does this mean that we need to stop using php mail in Email settings in Expression Engine? And specify an smtp server by default?