Hi Stefano! Thanks for your reply!
I tried with both SMTP and without and the problem persists. I contacted my host and they tried to change a few things but they can't find the cause and told me to contact you.
On the plugin's status the only problem seems to be CRONS:
Cron calls KO The blog cron system is NOT triggered enough often.
Trigger interval: average 599 s, max 5184 s, min 27 s
Could this be the reason? Or is this a separate issue?
Thanks again