What a uniquely quirky way to communicate! Glad to see via your sandbox update that you identified the cause.

Now all you need do is write "lambda-mta" to take over email!

Seriously - that 'free.fr' MTA spam block needs fixing if it triggers on proper 7-bit ASCII email that doesn't contain MIME parts (which are usually stuffed with HTML and/or images).

I use Mozilla Thunderbird in "Plain Text" mode. One benefit is there is no possibility to embed hidden malicious links in plain text. Another is small size (some embedded HTML emails are HUGE!).