Mac Mail - Incoming emails header and body confused

[Bazza1]

So recently on Mac Mail (MacOS 10.13.3), I have had cases where some email has come in (predominantly on my @icloud.com account) and either the From: line offers a different contact than actually produced the email, and/or the body of the email itself is scrambled / unformatted, and is from a different email. Also, some emails - already read and moved out of the Inbox within Mac Mail - have reappeared in there.

Meanwhile, no such issues with the Mail app on my iPhone, or Outlook on my Android tablet.

Thoughts?

 

[Bazza1]

With update to MacOS 10.13.4, I thought I'd give Mac Mail another try - and within days, I saw the same issues pop up - scrambled body text, often with a sender listed from another email. Oh, and my personal favourite (and one I'd forgotten) - when sending out a group email, it randomly adding a name from my Contact List to the list.

I've found others within Apple's own Forums with the same scrambled text / sender issue - and it seems to have kicked in within the last few High Sierra updates - but no one is certain what is going on or how to fix it. Apple, of course, is silent on the issue.

I've tried a Rebuild of my mailboxes and the other regularly posted fix - the manual Re-index of Mail via ~/Library/Mail/V2/MailData - but only thing in that folder is a .plist, and not the Envelope files I'm told I should find there.

So, sadly, back to the 3rd Party Mail apps.

(Currently with boring but functioning Thunderbird - no issues. Yet. Or just accessing my mail online.)