Email

https://tech.slashdot.org/story/15/10/19/1434236/the-hostile-email-landscape

Problem with email
As we consolidate on just a few major email services, it becomes more and more difficult to launch your own mail server. From the article: "Email perfectly embodies the spirit of the internet: independent mail hosts exchanging messages, no host more or less important than any other. Joining the network is as easy as installing Sendmail and slapping on an MX record. At least, that used to be the case. If you were to launch a new mail server right now, many networks would simply refuse to speak to you. The problem: reputation. ... Earlier this year I moved my personal email from Google Apps to a self-hosted server, with hopes of launching a paid mail service à la Fastmail on the same infrastructure. ... I had no issues sending to other servers running Postfix or Exim; SpamAssassin happily gave me a 0.0 score, but most big services and corporate mail servers were rejecting my mail, or flagging it as spam: Outlook.com accepted my email, but discarded it. GMail flagged me as spam. MimeCast put my mail into a perpetual greylist. Corporate networks using Microsoft's Online Exchange Protection bounced my mail."

Solution
Because of the (unbelievable) amount of spam hitting my server, I had taken out a Comodo AntiSpam Gateway subscription about two years earlier. It was initially free, but after a year or so they wanted money. Since the service rocks, I happily pay my ~$30 annually. What CASG also offers is outbound scanning: if I tell my server (an Exchange 2010 server) that the outbound smarthost is CASG, my email all of a sudden piggybacks Comodo's reputation. Voila, email flows without incident. Problem solved.