Here is a conversation between an MTA (I've been told this is "Windows Defender mail protection", but I don't know much about the remote system) and inbucket:

220 bitbucket.bentonvillek12.org Inbucket SMTP ready
EHLO GCC02-BL0-obe.outbound.protection.outlook.com
250-Great, let's get this show on the road
250-8BITMIME
250-AUTH PLAIN LOGIN
250 SIZE 10240000
MAIL FROM:<[email protected]> SIZE=38677 AUTH=<>
501 Was expecting MAIL arg syntax of FROM:<address>
QUIT

I don't know enough about SMTP to know who broke protocol here, but the remote system is in production use and sending many emails a day, so I'm guessing inbucket made the mistake.