Office 365 requires TLS encryption for SMTP. This isn't supported with IPO until RLS 9.1, where they add a "Starttls" check box, and TLS support in security settings.
You will either need them to setup a relay connector to forward the traffic to office 365 via relay, or use some type of public email relay server such as
However these do occasionally get blacklisted so you may have to create a new account for it once in a while if you go with the free version.
What my company did was build our own public relay server with SMTP2Go.com. You pay for the service, and then you can eithert use password authentication, or you can add their puplic IP address to the allowed senders list. It works fantastic, and its super easy to setup, and it supports standard and TLS on port 25, 80, 587, 2525, or you can use SSL on 443, 465, 8465. They do all the port mapping, and the nice thing is when you use a public relay, the customers are not limited to only using the work domain email account, and email address can be put in the users vm tab to receive voicemail emails.