Whoosh imports Tab-separated files, which need to be formatted in a particular way for Whoosh to recognize the fields.
It's very important that the first row of data has identifiers, and the most import of these is EMAIL, without this, Whoosh won't be able to find the addresses.
The other identifiers are important if you want to use the Whoosh 'mail merge' facilities. For example, if your identifier is ID then the mail merge placeholder will be __ID__.
Mail Merge is very useful if you want to incorporate any information specific to the email recipient in the email they receive. For example, if your imported file contains a "FIRSTNAME" column, in your email template you could type:
Dear __FIRSTNAME__,
Thanks for buying...
Then for every email that Whoosh sends out using SES, __FIRSTNAME__ will be replaced with the first name of your customer. You can use the Mail Merge feature to insert any information you like into an email, so long as the information is present in your imported file, with a column heading, and you use the placeholder in the email template.