Varnish / reverse proxy
Make sure that the useragent headers come along with it (for the spamcheck).
Error when trying to mail (sent an e-mail), but I still receive e-mails
Make sure that the sender is an e-mailaddress. Thus, the sender must not be a name, because that's not an e-mailaddress.
Tech info: http://wordpress.stackexchange.com/questions/131331/wp-mail-is-not-working-on-server
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