Setting up SMTP email results in error

[nMax]

I recently sat down to finally fix email notifications. SMTP is the way I usually do such things, and I was happy to see it as an option in the gui. However I cant get it to work, even though I believe I have set things up correctly I only get the following error (and no emails):

Could not test target: could not notify via endpoint(s): smtp-webmaster: network error: Resource temporarily unavailable (os error 11) (500)

My /etc/pve/notifications.cfg:

sendmail: mail-to-root
        comment Send mails to root@pam's email address
        disable true
        mailto-user root@pam

matcher: default-matcher
        comment Route all notifications to mail-to-root
        mode all
        target mail-to-root

smtp: smtp-webmaster
        from-address webmaster@example.com
        mailto-user root@pam
        mode starttls
        port 587
        server smtp.emailprovider.no
        username loginusername

with the correct password set up in /etc/pve/priv/notifications.cfg..

I did set up Datacenter -> settings: "email from address" to the same webmaster@example.com because I saw it in some forum thread..

What am I missing? Where can I find logs about this?