There is a script called lemmy easy deploy.
https://github.com/ubergeek77/Lemmy-Easy-Deploy/tree/main
You would setup the free plan for sendgrid.com
Their server setting are an ID, password, servernane and port you would put in the config file.
https://github.com/ubergeek77/Lemmy-Easy-Deploy/blob/main/config.env.example
I think what they meant by that statement is its not configured to setup postfix as an email server. When you edit the config file (prior to runnig the docker command to build it) youre setting lemmy up to use an external email service.