email sending speed – how to improve

koljanep asked:

I am maintaining a server to send email mailings (no spam, of course).
The current speed of this machine is approx. 2000 emails per hour.

(edit: actually I did a test mailing today, removing the throttling and sending a mailing to 2500 recent + active subscribers. It took approx. 1 hour and 45 minutes to get this mailed out.)

My boss pointed out that he is not satisfied with that, as he saw companies like Mail Chimp and the like where you can send thousands of emails within a couple of seconds / minutes. And they do go out, of course, as you get immediate replies, opens, etc.

My question is, what exactly is needed to achieve such a sending speed?
I mean, of course you can add to the hardware and build a more and more complex system of servers that all send out your mail, etc. And of course it is also a matter of having a clean list (no unknown hosts, etc. as they all eat up the resources of the server)

But aside from that, I am sure there must be other ways to improve this. Anyone can give some overview on this?


As requested in the comments, here are some more specifics on what hardware is being used as well as sending behavior:

Type of server

Operating system: CentOS Linux 5.11
Kernel and CPU: Linux 2.6.18-400.1.1.el5 on i686
Processor: Intel Core2 Duo CPU E7500 @ 2.93GHz, 2 cores
CPU load averages: 1.07 (1 min) 1.18 (5 mins) 0.65 (15 mins)
CPU usage: 4% user, 1% kernel, 56% IO, 38% idle
Real memory: 1.49 GB used, 1.94 GB total
Virtual memory: 1.13 GB used, 3.91 GB total
Local disk space: 55.10 GB used, 219.71 GB total


Postfix version 2.3.3

Average size of emails

Of the recent mailings, the largest one I found was just below 20k.
On average I can say it's probably between 8k and 10k per message.

bandwidth available

30Mbit/s symmetrical

Disk speed

hdparm -t /dev/sda

     Timing buffered disk reads:  336 MB in  3.01 seconds = 111.64 MB/sec

dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync

    16384+0 records in
    16384+0 records out
    1073741824 bytes (1.1 GB) copied, 23.9512 seconds, 44.8 MB/s

Measurements of various metrics

CPU load - see above

disk time - ???

RAM usage - see above

bandwidth usage - below data from "iftop -n" over a time period of ca. 5 minutes while sending mail.

    TX:      cum:  3.43MB   peak:  1.16Mb    rates:   5.36Kb  99.5Kb   137Kb
    RX:            1.01MB           120Kb             2.06Kb  38.0Kb  32.3Kb
    TOTAL:         4.44MB          1.28Mb             7.42Kb   137Kb   169Kb

Some maillog data:

in case this is of value, here are a couple of lines from the maillog:

Mar  4 14:00:32 mailserver postfix/smtp[25768]: 6C419107802A: to=<>,[]:25, delay=1.6, delays=0.05/0/0.14/1.4, dsn=2.0.0, status=sent (250 OK id=1YT8ud-0004fe-Rn)
Mar  4 14:00:32 mailserver postfix/qmgr[2806]: 6C419107802A: removed
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) lookup (score_sender), 1 matches for "", results: "."=>[Amavis::Lookup::RE=ARRAY(0xaa7f358),HASH(0xac5f891)]
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) lookup_re(""), no matches
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) query_keys:, myself@,,, .com, .
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) lookup_hash(, no matches
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) lookup (score_sender<>) => undef, "" does not match
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) SpamControl: calling spam scanner
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) spam_scan: DSPAM not available, skipping it
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) timer set to 320 s for SA (was 480 s)
Mar  4 14:00:32 mailserver amavis[26392]: (26392-01-46) calling SA parse, SA version 3.2.5
Mar  4 14:00:33 mailserver amavis[26392]: (26392-01-46) CALLING SA check
Mar  4 14:00:33 mailserver postfix/smtp[25767]: A5341207802D: to=<>,[]:25, delay=1.7, delays=0.15/0/0.14/1.4, dsn=2.0.0, status=sent (250 OK id=1YT8ue-0005BY-5x)
Mar  4 14:00:33 mailserver postfix/qmgr[2806]: A5341107802D: removed
Mar  4 14:00:34 mailserver postfix/smtp[25764]: C30371078144: to=<>,[]:25, delay=1.8, delays=0.05/0/0.13/1.6, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued on as 1425474034-NECyl5JAc9-0Xx8wjwN)

My answer:

Running a mailing list on a postfix server myself, I expect to see 2,000 messages get processed at least once (i.e. they might be deferred) before you can finish making a cup of coffee.

Your system appears to be running spam and virus scans on outgoing mail. While it’s a good idea to scan incoming mail, it’s probably not so good an idea to scan outgoing mail, especially if the source of that mail is already well-controlled. We can see that this is adding several seconds to the delivery of each message, and that it is also driving up your disk I/O dramatically.

I would reconfigure Postfix to stop scanning outgoing mail. If you have a need to scan outgoing mail normally, e.g. for the people in your office sending mail from their computers, then set up a dedicated mail server specifically for handling the outbound mailing list traffic.

View the full question and answer on Server Fault.

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