compatible with multisite?

From the FAQ:

Can this be configured globally for WordPress Multisite?

Yes, using the following constants that can be placed in wp-config.php:

MAILGUN_REGION       Type: string   Choices: 'us' or 'eu'
     ex. define('MAILGUN_REGION', 'us');
MAILGUN_USEAPI       Type: boolean  Choices: '0' or '1' (0 = false/no)
MAILGUN_APIKEY       Type: string
MAILGUN_DOMAIN       Type: string
MAILGUN_USERNAME     Type: string
MAILGUN_PASSWORD     Type: string
MAILGUN_SECURE       Type: boolean  Choices: '0' or '1' (0 = false/no)
MAILGUN_SECTYPE      Type: string   Choices: 'ssl' or 'tls'
MAILGUN_FROM_NAME    Type: string
MAILGUN_FROM_ADDRESS Type: string