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Developer Guide

Improving Email Deliverability


Outbox Endpoint

We recommend using our Outbox Endpoint Guide ] to send emails. It improves on deliverability and is integrated with webhooks and job status.

To optimize message deliverability with Nylas it's more important to have a few selective touches rather than blasting thousands of emails per day from an account. The most we've seen our customers successfully send per mail account per day is around 1000 emails, but the total volume depends on the user's mail provider.

Though, at those kind of volumes per account other factors are at play, like who you're sending to.

Unsolicited messages are more likely to raise flags versus emailing an address you already have an email history with. We recommend checking out Mailchimp's guide for common spam filter triggers. Also, you may want to consider something like 250OK

Gmail has some sending limits that they have shared here. For Exchange servers, it is dependent on how the Exchange administrator has configured the server.

More specifically, we recommend that you:

  1. Space out email sends. Sending an email every 30 seconds is a good rule of thumb.
  2. Back off exponentially after a message doesn't get through (i.e: you get a 429 error code from our API).
  3. Send a reasonable number of messages per day; we recommend at most 700, as per the rule of thumb above. Some providers like Gmail will lock out your user's account if they send more than 1000 emails a day, so it's definitely something to avoid.

These are the three basic rules we suggest, and we've seen many customers solve sending problems by following them. If you're curious about how Exchange throttling works behind the scenes, we recommend reading Understanding Client Throttling Policies by Microsoft.

Microsoft Outlook has a rigid filter system and a reputation system where if a specific email address is sending a lot of emails but does not get any replies it will get a lower reputation and will be tagged as junk/spam or if it is reported as such. One way to circumvent this is to be added as a safe sender or as part of a safe mailing list on the recipient's end. If emails are still being tagged as spam/junk after marking them as safe then a possible synchronization issue is at play; changing the account password should bring the account back to normal.

Updated about a month ago

Improving Email Deliverability

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