"Recipients can't access links" when message includes links to SharePoint files

As more of our users are including links to SharePoint-based files in their emails, we're seeing a spike in reports of the message shown below. All of the recipients of the message are members (some are external guests, but still "members") of the SharePoint site where the files are stored. I've been able to replicate this myself by using an alias of a user (rather than their "native" email/UPN) in the To field. The odd thing is that in that case, Outlook (in this case on the web) knows who the person is...it actually shows their profile photo...but somehow that doesn't resolve back to their proper identity. At least, not to the satisfaction of whatever code is checking the permissions on the files linked in the message. Is anyone else seeing this type of issue, and is there a fix? I realize that Outlook is trying to be "helpful", but it's supremely UNhelpful because of the rampant numbers of false positives.