Exchange smtp relay for Office 365 in non-hybrid mode.

Is there a way to use an on-prem Exchange 2010 server as a mail/smtp relay for Office 365 in a non-hybrid setup? I have a client who is wanting to migrate to Office 365, we are doing thins and its a very straight forward process for us except they have almost 100 appliances that all need to send email, such as printer scan to print, their phone system, and several other things of that type. They do not want a hybrid setup or to deal with the remediation process that's involved to get their system setup for it. What we and they are hoping is that we can use the current exchange server as a relay for these appliances without having to create a user account and buy a license for each. Part of the issue is the client doesnt have the time or IT man power resources to touch all these machines and update them, second is the additional cost for the extra license in O365. Sadly everything I've been able to find requires the appliances have a full user account or the on prem server be in a full hybrid setup. Is this accurate or, hopefully, have I missed something?