ah now it works, I had to activatae email routing on cloudflare. thank you!
the only thing I am not sure about now is that cloudflare replaced the TXT record for mailjet (v=spf1 include:spf.mailjet.com ?all) with this TXT record: v=spf1 include:_spf.mx.cloudflare.net
not sure but mailjet now suggest to change the TXT to this:
v=spf1 include:spf.mailjet.com include:_spf.mx.cloudflare.net ~all
does that make sense?