The Couch email sender is generating mails which are silently dropped by both Outlook and Gmail. I think those services have doubled down on their email strictness checks.

Anyone else having this issue? Anyone found a resolution?

This is an error chucked back to my server by Outlook for one of the emails that didn't arrive:

SMTP error from remote mail server after end of data: 550 5.1.20 SMTP; Multiple From addresses are not allowed without a Sender address

[KK]

SMTP Error 550 means the receiving mail server has refused to accept the message.
This can have several reasons e.g.
1. The sender's domain or IP address block is blacklisted
2. Authentication problem
3. The message fails to fulfill the receiver's policies (e.g. spam filters, restrictions etc.)
4. The recipient mailbox is does not exist (or is full).

TBH, for some of the reasons above there is little anyone can do to rectify things - IP address block being blacklisted is one which is very commonly encountered.
However, in your case, the error message mentions "Multiple From addresses are not allowed without a Sender address" - this would be point 3 above and this needs investigating.

I doubt you are specifying multiple email addresses as the 'from' parameter; something is not being set correctly which perhaps is causing malformed headers.

To debug the issue I suggest you please switch over to using PHPMailer as the email agent as shown here - viewtopic.php?f=5&t=10750&p=27240#p27240
(The particular configuration to use your web-host's local SMTP is given at viewtopic.php?f=4&t=13550#p39874).

The first link mentioned above has a section on debugging SMTP.
Following that, set the 'debug' param of <cms:send_mail> to '1'.
Try sending an email to the same host that flagged the error and then take a look at the log file.
It should give you a clue on what could be going wrong.

Hope this helps.

Thanks KK. Incredibly detailed response, much appreciated!