Don’t be lulled into the idea that great big rich companies automatically have high quality correctly configured Internet service. It is very common for outbound email service to be incorrectly configured.
A big part of the problem is that Microsoft email servers are configured incorrectly out of the box. They work correctly in the common cases and testing frequently stops there. At that point they will not use a backup or secondary email server AND they fail to keep things on a queue for a few hours to work around temporary failures as the standards dictate.
It is virtually impossible for your inbound email service to be completely down. There are 2 servers in different data centers capable of receiving mail. Yet, I commonly get reports of bounced email when our main mail server is off line. There is absolutely no excuse for that because the secondary is available.
Next time anyone, and especially a Great Big Company, tells you they couldn’t get an email to you, demand that they send you a copy of the mail with the failed headers. I can almost guarantee they will reveal a configuration error on their server. It’s only by pointing out their problems that they will become aware of and correct them. They should want to know they have a problem.