Exim ratelimit (padded to 15 characters)

I have a client who has been sending out about 100 emails to clients.
Bizarrely, he’s sending 100 separate emails, not one with lots of bcc
Anyway, he has had problems with only a few dozen going at a time. After asking a few times, I finally got him to try using Thunderbird instead of Microsoft clients, so we got a proper error message: and it was a ratelimit.
He seems to think that each hour he gets a few more messages out.
I looked at the docs, and they suggest putting a file in …domain/config with a number in it.
The thing is that my first attempt failed to have any effect, so I looked again.
The docs variously suggest that this file should be mailbox-ratelimit or mailbox_ratelimit. I have a suspicion that only one of these is correct. Which?
How does the ratelimit work? Is it a rolling 24 hours? Or is it from midnight to midnight?

It’s seems to be controlled by /etc/exim4/sympl.d/10-acl/60-acl-check-data.

There’s an explanatory comment or two in this file.

That says mailbox-ratelimit, so mailbox_ratelimit must be a documentation error.

Trouble is, I used mailbox-ratelimit and it didn’t seem to work.

Seems that the doings is in the line:

{([0-9]+)}{$1}{100}} / 1h / strict / $authenticated_id

Wish I understood it!

How is it limiting the email? The 1h suggests an hourly limit, but I can’t make sense of the bit before that.

It’s checking that it’s read something that looks like a number from the config file, if it hasn’t it defaults to 100.

The file is significant – mailboxes/[user]/ratelimit takes precedence over any domain-wide config/mailbox_ratelimit