A segment is a subset of one of your existing subscriber lists. For example, instead of emailing everyone on your 'Widget Fans' list, you might only want to send an email to subscribers who live in the United States. That's a perfect job for a segment.
Segments can be based on any default fields in your list (e.g. 'name', 'email address') as well as any custom fields you create for the list (e.g. 'gender', 'favorite products', 'email frequency preference').
You can also create segments based on campaign activity. For example, you might want to see which subscribers clicked links related to commercial widgets versus widgets for the home so you can segment based on content interests.
But let's start with the basics. Below are instructions for building a segment based on the default field, 'date subscribed'.
Create a subscriber list segment
- From the Lists & Subscribers tab, open the list you want to create a segment for and then click Segments in the right sidebar.
- On the "Manage segments" page you'll see any segments that have been created for this list. To make a new one, click Create a new segment.
- Next, type in the Name field something to summarize the type of subscribers this segment includes. Let's say you want to use a segment to find people who subscribed after your last campaign was sent (because it announced a huge sale and you want new subscribers to know, too). Name it "Subscribers since Nov campaign" and click Start building segment.
- Now you need to create the rule that will separate subscribers on the list before your November campaign was sent, from those who have signed up since. Click the drop-down menu labelled "Create your first rule based on..." and select Date subscribed, then click Add rule.

- You also need to specify the 'rule condition' (which we'll explain in more detail further down). As shown below, there are 4 rule conditions for date fields: Equals, Does not equal, Is before, and - the one we want for this demo - Is after.

- Finally, set the day/month/year menus to the date your last campaign was sent (our example date is November 5, 2012). For this kind of segment, only 1 rule is required so click Save and refresh count to finish.
When a new segment is created the system finds every person on the subscriber list who matches the rule/s you specified, and the total number of subscribers is displayed in the right sidebar (as shown below).

You can combine any number of rules to build a segment, although not all "conditions" are available for every rule.
Conditions available for:
Email address:
- Equals
- Does not equal
- Contains
- Does not contain
Date fields:
- Equals
- Does not equal
- Is before
- Is after
Campaign activity:
- Campaign was opened
- Campaign was opened - Any link clicked
- Campaign was opened - Specific link clicked
- Campaign was opened - No link clicked
- Campaign was not opened
Text fields:
- Equals
- Does not equal
- Is provided
- Is not provided
Number fields:
- Equals
- Does not equal
- Is greater than
- Is less than
Note For segments based on campaign activity (e.g. grouping all subscribers who opened your last campaign and clicked a link) you must create the segment after the campaign has been sent.
Segments are smart!
Next time you create a new campaign and get to the "select recipients" part, your new segment will show up as an option:

Once you've created a segment it will keep itself updated as your list grows/changes. When a new subscriber comes along who matches the segment's rule/s (e.g. the subscriber is female, lives in Baltimore, is interested in widgets for the home) they are automatically included.
Similarly, subscribers who no longer match the the segment rules - for example by becoming inactive or updating their details - are automatically removed and will no longer be included in the segment.
FAQs
Is sending to a segment charged like a normal campaign?
Yes, each time you send to a segment of your list, you will pay the normal rates for a campaign.
Can one subscriber be on more than one segment in my list?
Yes, if that person fits the rules for multiple segments that have been created. But no matter how many segments you send to in a single campaign, each subscriber will only get one email (i.e. you don't need to worry about anyone receiving duplicate emails).
Do I have to update them manually?
No - segments are automatically updated before you send to them, and each time you view the segment details. They don't update 'live' because of the amount of processing that would require.
How do I import a subscriber list into a segment?
There's no way to manually add or import people into a segment; you just need to make sure that the new people meet the rules setup for your target segment and that segment will automatically include them.
If you wanted to setup a segment of specific subscribers, you'd need to add a custom field that you can use to identify them. For example, add a "friends" field, and add a value of "yes" for each of the people you want to email separately.
Can I do a 'contains' rule for custom fields?
Sorry, no. At the moment, you can only use 'Contains' and 'Does not contain' rules for the email address.