There is nothing intrinsically that will cause group emails to be regarded as spam. Make sure you send the email through the web interface and in a manner where the standard high reputation Fastmail SMP servers are used. Ensure you stay within the hourly and daily limits on number of outgoing messages (each recipient counts as a separate message). These days, those limits are in the thousands, so regular group emails should not be an issue (see
https://www.fastmail.com/help/accoun...s.html#sending ).