Segments work by referencing data fields according to the type of data they hold. It is therefore critical to pick the right field type. 

Do this based on how you want to search for the values, not what the values are.

For example, you might have numerical customer IDs, but unless you actually want to treat them as numbers (e.g. using conditions such as greater than or less than), you should use a text field for them. This will prevent any problems that might stem from the fact that leading zeros are ignored by numeric fields.

You always need to include at least one criterion for a segment to work. 

Even if the segment is based on all contacts or on a specific contact list, it still needs at least one valid criterion to return any contacts.

Due to the flexibility of this feature, if you are experimenting with different criteria it can happen that the Save button becomes available without any valid criteria being present. 

If you find your segment is not returning any contacts, this is the first thing that you should check.

Make sure you know exactly what each field means. You can find out about them here: The Emarsys system fields.

Common misunderstandings include:

• Email valid
  This applies to bounces only. This field is set to YES for all new contacts, and this field is only set to NO after a hard bounce or a number of soft bounces.
  This field has nothing to do with email syntax. All contacts with unusable email addresses, or even with no email addresses, will still have email valid = YES field value because they cannot be bounced. 
• Phone
  This is a short text field for a contact phone number and has nothing to do with mobile marketing. If you want to use Mobile Engage or SMS then you must create another field called Mobile which will contain the mobile phone numbers that you will use for your campaigns.

If you want to create a segment containing all the contacts in your database, you need to create a pair of mutually exclusive criteria that cover all possibilities. For example:

• Email is not empty OR Email is empty

This will ensure that every single contact entry in your database is in the segment.

What are you using the segment for?

If you are going to use it as the basis for a campaign launch list, why not make sure that it only contains relevant contacts by starting with the most important criteria. For example:

For email launch lists:

• Email is not empty
• Email valid equals yes

For Mobile Engage or SMS launch lists

• Mobile is not empty

Anyone who fails to qualify for any of these cannot receive a campaign anyway, so now you can keep your launch lists more focused.