Setting up SMTP email results in error

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:

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