iOS 11.4.1 Mail app w/ Exchange account keeps crashing “8badf00d”
Is this possibly due to a timeout issue?


The 0x8badf00d error code basically means an application is taking too long, so you're on the right track in terms of the most likely problem (i.e. it being a timeout issue).


  I'm wondering is it something in my Exchange inbox that's causing the issue and if I can find out what it is, will deleting it help?


That certainly could explain it, although with 300K emails it may not be so easy to identify the culprit.

As a workaround/test you could try using another email client (such as MS Outlook) to see if that also encounters the same issue. 

Also, if all your emails are in your inbox, you may want to try creating some Outlook Data Files (which basically appear as folders or separate mailboxes) to move bulk amounts of your email to to see if that helps you isolate the problem.

[UPDATE]

Apple's Developer Technical Q&amp;A QA1693: Synchronous Networking On The Main Thread provides some additional info on the 0x8badf00d error code, although this is an archived document written for Developers.

I suspect the issue you're having is a little more involved (e.g. Mail may be performing other tasks that Outlook isn't, etc), and is therefore more sensitive to any potential problems with your MS Exchange data. I know that many organisations are even rolling out VMware's Boxer app because of issues accessing MS Exchange Server accounts on mobile devices.