I do not use a relay server for sending e-mail. Is it normal that it uses the relay=deurenbergmedia-nl.mail.protection.outlook.com?
The hosting company tells me they do not block any ports in the firewall. My own firewall (in plesk) also doesn't block anything.
I have this exact same issue, but only for the outgoing mails. It happened Since 18.0.56 update 1 and there are no errors that lead me to the problem. All outgoing mails are sent to the postfix queue. SPF/DKIM/DMARC/SMTP/port 25 open. Everything checks out at mxtoolbox. I can send mail to...