WordPress installations are not sending emails - need fully-qualified hostname

Oh you raised an interesting point @hmaesta ! i am using my registrar to send and receive mails.

MX Lookup for let.de:

Target	IP	Preference value	TTL
spool.mail.gandi.net	217.xx.xxx.x	10	4 min 59 sec
fb.mail.gandi.net	217.xx.xxx.xxx	50	4 min 59 sec

Would this change anything regarding my config ?

Thanks