Automatic reply: mailbox unavailable

You can always try and see if it works, but I think it's more likely that Hotmail is blacklisting free, and in that case, there's not much you can do.

Here is what the message means

Some recipients did not receive your message.
You will find attached the message that could not be delivered.
We invite you to check the recipients' addresses and try to
resend the message.

You are probably on Free and your recipient on Hotmail.
This is due to Hotmail refusing emails from Free that are blacklisted..

Make sure you are properly
authenticated as the sender. Otherwise, rejection by
Hotmail (and others).

Except that I don't see the point in putting myself on a blacklist since I don't know her. Besides, at that time it was for a sale so I think it wasn't in her interest to block my response.
Thank you anyway, hotmail addresses sometimes go through and sometimes they don't, so I deal with it.
See you soon.

I found this on a site about Outlook: it's good

with Outlook!

How to decline the sender's read receipt by default? [top]

author: Morgan BILLY
You can decline to send a read receipt to recipients who request one with each send:
Menu Tools -> Options -> Preferences tab -> Mail Options -> Tracking Options... -> Check Never send a response -> confirm with Ok in all windows.

Hi,
Have you checked that the address is correct?

Ah yes, I understand you, it's clear.

After reflection, I think this person used Outlook through Free, and I can't tell how; personally, I don't know how to configure Outlook. I keep wondering how he managed to read my emails without me receiving the read receipts from Free. Instead, I receive messages telling me that his email address does not exist, even though I'm used to writing to him. I had this problem before our big argument, but only sometimes, and now I'm thinking that he must have used his strategy to block me before, and that saddens me a lot.
Anyway, that's how it is. But I recognize that he is very clever in everything. He is a master's student in computer science.

It's rather the person you're sending to who has put you on their blacklist and has their email system refuse to let your emails go into their inbox. This is my case; I'm with Free and I've sent an email to a person I regularly send and receive emails from, who is also with Free, and after a dispute, they allowed themselves to manipulate their email system to refuse all my emails. And this is what I receive:

This is the mail system at host smtp4-g21.free.fr.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system