You can get round this by 'smarthosting' your outgoing mail. Typically this is your ISPs SMTP relay server. For example, I host my own Mail environment (MS Exch

You can get round this by 'smarthosting' your outgoing mail. Typically this is your ISPs SMTP relay server. For example, I host my own Mail environment (MS Exchange, not that it's relevant) and all my outgoing mail is sent directly via SMTP to my ISPs smarthost. That smarthost then directs it where it needs to go based on the recipients domain. This way you are not sending mail directly from your 'untrusted' mail server, into say, Google or Yahoo. I know from experience that I have to smarthost, as Gmail and Hotmail/Outlook.com reject silently any mail that come direct from my mail server. It's a pain, but there are solutions out there.