What is the CSV file format to import leads/subscribers properly?

MailBluster doesn’t force you to upload your CSV file in any particular format, rather it’s flexible with any CSV file.  You can select which column contains what data. For example – you can have a column in your CSV named  “Subscriber name” and select it column as “First name” while importing in MailBluster.
Here is a sample CSV file with columns necessary for uploading leads to MailBluster. You can follow this to upload your data.

See the sample file on Google Sheets.