Move configuration options into central configuration file
The plugin needs to be configured to run with the following credentials to authorize its requests against the Sendgrid API.
dm4.sendgrid.system_mailbox=mail@domain.tld
dm4.sendgrid.from_mailbox=mailte@domain.tld
dm4.sendgrid.from_name=System Mail
dm4.sendgrid.api_key=XYZ
Otherwise this plugin will not work. This plugin is particular groundbreaking/useful if one has to operate the sign-up plugin without an smtp server. Mail sending with this plugin is done via HTTPS.