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What is the best practice to send an email to a large number of users?

I am developing a plugin for the website of an association and one of the things needed is to be able to send an email to all (or a specific subset) of the members of the association.

This will be a fairly rare thing to happen, but when it happens 500 to 1000 emails are expected to be sent.

Now, I can think of several options:

  1. Send each email separately
  2. Send email in blocks of 50/100
  3. Send one single email for everyone

Option 1 is nice because I can personalize the email (Dear Name Surname, ....), however we can live with a generic email for these situations.

  • What are the drawbacks of each of those options?
  • Can you advise any alternative strategy? I was going to simply use wp_mail but maybe relying on an external service (which one?) is better? I would like to avoid setting up a mailing list on an external server, I don't want to have two different databases with user info
  • Is there a way to send the emails at regular intervals maybe using wp_cron? How would you go about it?

Any further advice is very welcome.

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Sending mass emails that actually get delivered is not a simple matter at all. Even if your code sends 1000 messages correctly, many if not most of them will get blocked or labeled as spam unless you comply with a long list of mass email best practices. (See also: The FTC's CAN-SPAM Act - Compliance Guide.)

I recommend that you use (or study the code of) a well-established newsletter plugin with a high rating, as these are more likely to employ best practices for deliverability. You might also install a plugin to send outgoing emails via Mandrill or AmazonSES. (With Mandrill, you can send up to 12,000 emails a month for free.) These services have established relationships with ISPs and spam blacklisting services, so they can better assure the deliverability of your mass emails.

  • This answer does not specify which of the three options mentioned by nico above is best, though. I set up an Eleastic Email account and installed their plugin, which hooks to wp_mail. But I'm not sure if it is better to have a big loop with an individual call to wp_mail for each user (options 1 or 3), or a single call to wp_mail targeting all users, or something in between (option 2). Any idea? – Franco Aug 12 '18 at 8:46

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