Monitoring Your Mail Stream


One of the most important things for any mail sender to do is monitor their mail stream. There are a number of things that every mailer should pay attention to.  Some are things to monitor during delivery, some are things to monitor after delivery. All of these things tell senders important information about how their mail is being received by their recipients and the ISPs.

What to monitor during a send?

Many modern, commercial MTAs incorporate dashboard monitors into their systems so that senders can monitor sends as they are going out. Likewise, ESPs also have dashboards that give an overview of the progress of a send. These dashboards are a little different across installations but most will show you the active queue and the delayed queue. It can be useful to keep an eye on these values during a send.  are only two values that are useful to keep an eye on.

  • Active Queue size. The active queue is for messages received by the MTA and have not been sent yet. The number of mails in the active queue tells you how fast the send is progressing and how many messages have been sent.
  • Delayed Queue size. The number of mails in the delayed queue tells you how many messages have been sent but were temporarily refused by the receiving mail server (4xx response). Mails in the delayed queue may indicate some spam blocking or may be related to a problem on the receiving end.

What to monitor after a send

After a send there are a lot more things to monitor. Some of these things tell you about delivery and some of them tell you about how well a mail met user expectations and some tell you about what the ISPs are seeing from your send.

  • Feedback loop messages. The FBL messages tell you which of your recipients saw a message in their inbox and decided the mail was spam. For small volumes of mail this can be very noisy and not accurately represent how much users liked (or didn’t like) a particular mailing. Experience tells us, however, that for larger volumes of mail FBLs are an accurate measure of user reactions. High numbers of FBL messages, for mail with high inbox delivery, says that this was not something the users liked.
    • Pros
      • Accurate measure of user response for large numbers of recipients.
      • Tells you exactly what the user thought about the email.
      • One of the metrics ISPs use to make filtering decisions.
    • Cons
      • Very noisy for low volumes of mail.
      • Only some ISPs offer FBLs (see ISP Information for details)
      • Some ISPs do not share every FBL messages.
      • Need to know inbox delivery to be able to accurately determine complaint %.
  • Hotmail Smart Network Data Services. Microsoft provides senders with data about mail from specific IP addresses. Users need to sign in and authenticate the IP addresses, usually through clicking on a link to the registered owner of the IP range.
    • Pros
      • Gives number of spam trap hits per IP.
      • Gives number of complaints per IP.
    • Cons
      • Red/yellow/green color coding that sometimes doesn’t correlate with actual delivery.
      • No historic data.
  • Google Postmaster Service. The Google postmaster pages are new but provide data for an authenticated mail stream. It’s almost too new to give pros and cons.
    • Pros
      • Gives reputation data for IPs in high/medium/low/poor.
      • Gives reputation data for domains.
    • Cons
      • No API access.
      • No way to download data.
      • Some people sign up but never get any data and aren’t told why.
  • Open. Open rates are not an accurate measure of how many people actually read mail, as they require users to load images and not everyone loads images. However, they are a good trending measure and can be used to compare different mailings to the same group of people.
    • Pros
      • Easy data to get with modern sending infrastructure.
      • Every ESP will provide open data.
      • Trends can show how people react to mail over time.
    • Cons
      • Not always accurate.
      • Open rates can vary depending on target market.
  • Clicks. Tells you how many people were interested enough in your mail to click through.
    • Pros
      • This is what you want! You got people to click through and look at your website.
    • Cons
      • ISPs don’t actually measure clicks so don’t count towards ISP engagement data.
  • Unsubscribes. Unsubscribe rate tells you how many people so disliked your mail that they wanted no more mail from you. It’s a bit of an unconventional measure, but I like using it to see how recipients are responding to mail.
    • Pros
      • A form of negative feedback that doesn’t affect deliverability. ISPs don’t know when someone has unsubscribed using your link.
      • Useful as trending data.
    • Cons
      • Some MTAs don’t provide easy access to unsubscribe numbers.

There’s no such thing as a useless source of data. The more data and information you have, the better decisions you can make. Creating effective views of the above data sources can lead to much more direct and immediate feedback on how recipients are reacting to your mailings. This knowledge can drive more effective and productive emails in the future.

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  • Very nice summary. I would think that unsubscribes using the list-unsubscribe through those webmail providers that embed them into their interface is actually monitored. Anyway, one of the most interesting ratios to monitor is the unsubscribe to CTR, as it may be very revealing.

  • Hey Laura-
    Great writeup! This should serve as a core checklist for all mailers.
    One thing I’d recommend you add is … “Replies” . I always find that monitoring the content (and tone) of recipient’s responses often serves as a good litmus test of the success of the mailing, as well as revealing any actionable items, e.g. broken links, data errors, etc.

  • Hi,
    you forgot several highly important free sources of data:
    1. and consoles: assuming you have enough sending volume to these domains, the consoles offer the most absolutely detailed data, as postmaster consoles go. actual placement and engagement rates. make sure to use the headers they expect, to make the most of the service.
    2. blacklists/whitelists: after big sends, head over to after a few hours, see if you got blacklisted or lost a whitelist.
    3. online reputation sources (RP, WG doesn’t update often, but even little changes can hint towards a problematic mail stream or snoeshow spamming in hiding.
    4. SMTP log analysis: some SMTP messages are verbose enough to understand. YMMV between major providers, at the time of writing google is highly verbose, hotmail/outlook is highly secretive, tending to reply ‘mailbox is unavailable’ for just about everything and yahoo responds with error codes which have spotty documentation.

By laura

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