Several of the 39 custom fields are set up by default (and you can certainly edit them), and others can be added as shown below.
To locate Recipient fields in the admin console, go to Settings > Account settings > Recipient fields
- Field ID: a numeric identifier
- Field name: choose a name that easily defines which kind of data the field contains
- Default value*: is used when a message is sent, if the field is inserted into the message as a dynamic field. If the field is empty for one or more recipients, the default value is used when the message is sent out to those recipients.
- Dynamic field (Y/N): if set to Yes, the field will be available in the list of fields that can be inserted into a message using the corresponding feature in the message editor when adding or editing a message
- Required field (Y/N): if set to Yes, the field will be considered a required field on subscription forms that use the default form field validation code that is included in MailUp.
* The Default value:
- you can use them to add dynamic tags to your messages (e,g. Dear and the customer's first name).
- you can map them to a import file's columns when importing data into MailUp
- you can use them on subscription forms and profile update forms to collect additional information on your subscribers
- you can take advantage of them when integrating MailUp with other applications
The official answer is complex because it depends on the encoding used for that list.
1) Each recipient field has a maximum length of 200 bytes
2) By default encoding set to a list is UTF-8, which uses from 1 to 4 bytes per character
...in most cases 100 characters work in the recipient field