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One thing I've been doing with my mailing list is using custom fields to target specific mail to specific users. The custom fields and users are all created using the api. One issue I encountered however is that the segments don't appear to update on their own. A user can be created and match a segment, but even when sending a campaign, they'll be excluded until I manually go and refresh the segment.
Is there any way to automate refreshing a segment?
You can create a cron job to update segments automatically. See the bottom of the page for cron job setup instructions when inside a segment: