Does your email have to say "Sent with Mixmax"? No — you can stop this line from being added to the emails you send. While "Sent with Mixmax" is enabled by default, you can remove it when you purchase any of our paid plans. It cannot be removed when you're on the Free or a trial plan.
To remove it, go to your Signatures Settings (Settings > Personal tab > Signatures) and untick the box for Include "Sent with Mixmax."
