My main email address is a hotmail address, in use since >18 years.
These mails are forwarded to my fastmail account for reading and replying.
For the last 10 years or so, I use a hotmail identity in the fastmail web interface for replying; it has the smtp.live.com server configured so that emails are sent using microsoft servers. (otherwise, using fastmail smtp server, some hotmail recipients had my mail marked as 'suspicious' starting some years ago).
However, recently I received twice a bounce message from fastmail, indicating issues:
<some-recipient@address.com>: host 52.96.90.50[52.96.90.50] said: 432 4.3.2 Concurrent
connections limit exceeded. Visit
https://aka.ms/concurrent_sending for
more information. [Hostname=AM0PR0102MB3474.eurprd01.prod.exchangelabs.com]
... <some more stuff> ...
It was helpfully diagnosed by the fastmail support people as follows:
Quote:
It appears Microsoft is now throttling the total number of SMTP connections at a time:
"Under the new limit, up to three concurrent connections are allowed to send email messages at the same time. If an application tries to send more than three messages at the same time by using multiple connections, each connection will receive the following error message:"
So, when you attempt to send emails using your hotmail address to multiple recipients, I think if the recipient count is more than 3 addresses, Microsoft deferres the delivery for a while for some of the recipients.
[...]
I am not sure if there is anything we [fastmail] can do about this, but I'll make sure to bring this to the attention of our engineers. Meanwhile, I think when using your hotmail address, sending emails to 3 or lesser recipients at a time would probably avoid this.
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So it looks like this is potentially bad news for me (and people with a similar setup); breaking daily & reliable use of a hotmail identity for sending from the fastmail web interface.
If anybody has any clue or approach to reliably keep using my hotmail/fastmail setup, I'd be glad to hear.
(BTW: I know the setup is not ideal and I should switch to a custom domain etc, but I'd like to keep using my hotmail address as main identify, as that's easy for my contacts)