Emails sent to Hotmail are vanishing

I'm using the CI email library to send personalized emails to a group of about 30 people. Some of them have Hotmail. They don't receive the emails. I know this is not really a CI issue, but does anyone have an idea how to solve this?
In the sending proces, all goes well. I've tried an email with both a Hotmail address and my own Gmail address. I'm getting the email correctly, but the Hotmail recipient doesn't.

Hotmail will block your servers ip really fast if it thinks its spamming (could be someone who is on the same server).

They don't even appear in your spam folder?
If they do you can right click on them and view original message, so that you can look for spf, dkim and dmarc.
You will also se your email servers SCL rating. Don't know the exact value for spam or deletion. Lower = Better.

If you can't see the email at all, check these on another e-mail service and see how they are reacting.

I tried to send the email to my own @live.nl address too. Same thing happend. On Gmail, the email is received. On Microsoft's Outlook.com, the e-mail sent to my @live.nl address is neither in my inbox, nor in my spam folder. It simply disappeared.
Before sending the mail, I added the domain to the list of trusted senders.
What makes it even more mysterious, is that other emails from the same domain and with the same email header, are received on hotmail.com and live.*
It must be something in the subject or in the content of the mail that triggers Microsoft to mark the message as suspicious.
But I don't even get the chance to unmark it as spam. The email is vanished in mid air.

The way that mail services define what is spam and what is not spam is very complex. If you were sending email from a server using SMTP, and the static IP address of the server was for only the one domain and nothing else, then you have a better chance of email delivery.

In my own emailing adventures (usually on shared servers), I've found that SMTP is a must, and that if possible relaying all email through a gmail account helps ensure delivery.

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How does the spf, dkim and dmarc look on gmail?
Have you changed any specific email preferences? Like priority?
Do you have any smiles in your subject?

Can you try and send it with another library e.g. PHPMailer so we know it's your servers fault and not a specific CI setting.
Only Microsoft knows of any trigger words, but somehow you spammed the same text too fast. Or someone else use the same phrase.

How does the spf, dkim and dmarc look on gmail?

How can I check these?

Have you changed any specific email preferences? Like priority?

No, I haven't.

Do you have any smiles in your subject?

No, I don't.

Can you try and send it with another library e.g. PHPMailer so we know it's your servers fault and not a specific CI setting.

Like I said, sending the mail is not the problem. The same e-mail, sent to a Gmail address, is received ok. If I use 2 recipients in the "To" field, the mail is received by the Gmail recipient, but not by the hotmail or live recipient.

1. Press the little arrow next to the reply button on Gmail -> Show original.
2. Ok
3. Ok
4. They do have other default settings, like script name etc. If outlook.com don't like that.

I use two methods to work out email delivery problems.

First is to check the 'spammyness' of my emails with https://www.mail-tester.com
It gives your message/server a score out of 10, but breaks down the reason for that. I was getting below 3 on my message/server combination, but am now usually 9 or above.

Second, I use an account with SendGrid (https://sendgrid.com) to send my mail rather than rely on my own server. They have a free trial. The cool thing for me is that SendGrid tells you why your emails were bounced/blocked. Lots of other handy stuff too.

Get another SMTP service.