This actually seem to have started at least early in December. Microsoft (Hotmail) seemed to block all incoming mail from Linode, without alerting the recipient or routing to the spam folder.
Looks like the problem is still afflicting Linode customers.
Microsoft appears to have delivered the unwanted Christmas gift of email blocklisting to Linode IP addresses, and two weeks into 2022 the company does not seem ready to relent. Problems started as large chunks of the world began packing up for the festive period. Complaints cropped up on Linode's support forums when customers began encountering problems sending email to Microsoft 365 accounts from their own email servers.
More recently, the Linode team has offered to swap out affected IPv4 addresses for unaffected ones – or, for a fee, it will add some new ones to users faced with the problem. "While we cannot control how long it takes for Microsoft to address the issues on their end," said Linode, "we do have potential solutions that we can offer in order to help customers avoid the current 'Banned Sender' bounces."
Blocklisting IP addresses to prevent the delivery of unwanted emails is not a particularly complicated concept, although Microsoft has perhaps been a little more enthusiastic about this than is strictly necessary over the years. In 2019, tsoHost's bulk email domain found itself on the naughty step for Outlook and Hotmail addresses and getting itself off again proved a bit of a challenge.
Linode itself is an infrastructure-as-a-service outfit, with data centres spread around the world. One can host one's applications (including email services) and data on its platform as an alternative to the bigger boys. Right up until Microsoft decides to slap the IP addresses one is sending from on to a blocklist.
source https://www.theregister.com/2022/01/17/linode_microsoft_email/ [theregister.com]
special thanks to martyb, janrinok, FatPhil who helped me get back on SoylentNews