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Old 3 Aug 2021, 09:42 PM   #1
rabarberski
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Join Date: Nov 2006
Location: Ghent, Belgium
Posts: 1,027
Sending from fastmail via hotmail smtp no longer reliable for daily use?

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:

<[email protected]>: 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.
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)
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Old 4 Aug 2021, 01:57 AM   #2
JeremyNicoll
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Location: Scotland
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Those three connections are presumaby between one of FM's servers and the MS one, or - even worse - between a single FM server (maybe always the same one) and the MS one.

I wonder if FM are certain that the count only applies to connections being made to send your traffic? I would have expected MS to be counting any/all connections from FM and thus the problem to occur if more than three of FM's customers' setups were simultaneously trying to send mail that way.
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Old 4 Aug 2021, 04:43 AM   #3
rabarberski
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Quote:
Originally Posted by JeremyNicoll View Post
I would have expected MS to be counting any/all connections from FM and thus the problem to occur if more than three of FM's customers' setups were simultaneously trying to send mail that way.
I agree. That were my first thoughts also.
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Old 4 Aug 2021, 07:05 AM   #4
emoore
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Join Date: Apr 2002
Posts: 259
Microsoft Exchange documentation at https://docs.microsoft.com/en-us/exc...n-improvements states :

"New throttling limit for concurrent connections that submit messages
The service has various limits to prevent abuse and to ensure fair use. An additional limit is being added. 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:

432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded

Additional throttling limits for the SMTP Authentication protocol are:

30 messages per minute
Recipient rate limit of 10,000 recipients per day
Exceeding these limits will cause the following issues:

Exceeding the per minute limit causes email delivery delays, any excess in message submission will be throttled and successively carried over to the following minutes. Exceeding the per day limit causes the following error message:

554 5.2.0 STOREDRV.Submission.Exception:SubmissionQuotaExceededException"

Is Fastmail forwarding to Microsoft servers popular enough to get 10,000 recipients a day?
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Old 4 Aug 2021, 08:21 AM   #5
BritTim
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Join Date: May 2003
Location: mostly in Thailand
Posts: 2,911
In your situation, I would get my own personal domain, and
  • still accept emails sent to the Microsoft account;
  • reply using an email address of my own domain, using Fastmail SMTP;
  • optionally use auto bcc to save a copy of outgoing emails to the Microsoft account if you consider this worthwhile;
  • have an aim of switching correspondents to consider your own domain email as your primary email address over a period of two years or so; and
  • probably plan to retire your Microsoft account if it is a paid account within about five years or so.

Years ago, you could simply have used the Fastmail servers to send on behalf of your Microsoft account. However, today that is likely to run into problems with things like DMARC checking.
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Old 5 Aug 2021, 03:01 PM   #6
hadaso
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Quote:
Originally Posted by BritTim View Post
Years ago, you could simply have used the Fastmail servers to send on behalf of your Microsoft account. However, today that is likely to run into problems with things like DMARC checking.
Years ago you could send email from Fastmail with different email addresses in the From header and "envelope from" by configuring an identity with the two addresses. So perhaps back then you could send form Fastmail using a Microsoft (hotmail, live, outlook) address in the from field and an address in a Fastmail domain in the envelope from, and perhaps this would not be rejected by Microsoft's SMTP servers.
The option to set a separate envelope frpm in an identity was removed several years ago.
It is still possible to send an email from Fastmail with different addresses in the "From" header and envelope from. I send email from Fastmail using my work email address, and never noticed any problem with it except when the recipient address is in my employer's domain. Our server refuses mail coming from outside using our domain in the envelope from. Sometimes I forget this and send mail this way (and sometimes I do it intentionally). Of course it bounces. Then I just go to the sent message and use "send a copy" to send it again to the same recipient, and this time it goes through because the "send a copy" action uses the address from the default personality as the envelope from (and doesn't change any header in the original message).
So when Microsoft bounces mail, resending it this way would let it go through.
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