On a forum I migrated, a lot of users use a @free.fr email.
Free is a major Internet and mobile provider in France.
I wonder why there are so many bouncing emails (and therefore revoked emails of active users 
)
There are not only @free.fr emails, but the majority. Other emails are also emails from other French Internet providers.
I use Mailgun. I had no issue before with my other French Discourse, it’s specific to this one.
Any idea what could trigger this? 
Are you able to check the logs of the party you’re using for email sending? They might reveal the cause.
Perhaps you should check that your Mailgun account is properly setup to match your SPF/DKIM/DMARC records.
Here’s a easy tool: https://www.mail-tester.com/
My domain on Mailgun is well configured, including SPF & DKIM.
Logs on Mailgun for most @free.fr addresses return “spam detected”; it seems to be a common issue with them.
Other providers have more or less similar reasons. @laposte.net addresses return “service refusé” with an error code that might or might not be related to spam detection.
For some reason, ISP email providers seem to be a bit sensitive regarding the emails they receive… I’ll guess it’s normal and that they’re just maybe… Bad at what they’re doing?
cellule.abuse@laposte.net is quite reactive, you would have to use their contact form and answer no at the end:
never heard back from free though (half of the emails from free users are flagged by gmail 
I don’t know how they handle email over there…)
