[Koha-bugs] [Bug 30013] New: Limit/throttle SMTP connections

[bugzilla-daemon at bugs.koha-community.org]

https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=30013

            Bug ID: 30013
           Summary: Limit/throttle SMTP connections
 Change sponsored?: ---
           Product: Koha
           Version: master
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5 - low
         Component: Notices
          Assignee: koha-bugs at lists.koha-community.org
          Reporter: magnus at libriotech.no
        QA Contact: testopia at bugs.koha-community.org

We have a setup where Koha is sending email through the SMTP server of the
library's mother institution. We are seeing a problem with sending lots of
emails in one go, where the first emails are sent ok, but then the rest fail
with an error like this:

unable to establish SMTP connection to (mail.x.y) port 25

It looks like this is due to some limit on the server, making it unresponsive
if there are too many connections from the same source in a short time. 

I don't know a lot about SMTP but I can imagine a couple of solutions:

- Introduce a configurable "sleep" period between each call to the external
SMTP server (or batches of a configurable number, with a configurable sleep
between each batch) 

- Looking at the response from the server and if "unable to establish SMTP
connection" is received, wait for a configurable amount of time before trying
to resend the failed message and the rest of the queue

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