Why am I told, when importing a .csv file, that the emails are not valid, when I know they are?

This is usually caused by not having the correct headings in Row 1 of your .csv file. They should be:   first_name   last_name   email

This is because I have a database ready waiting for your data, with exactly these labels.

The order of the three columns isn’t critical, as long as the data corresponds to these headings.