Too many recipients error while sending email newsletter using Mailgun

Hi, I get this error whenever I try to send newsletter. Can anybody please help me to fix this?
Capture

Have you set up a payment method in your Mailgun account?

You may get more details about the error if you look in your server side logs, there are details for how to do that in the docs:

Yes, I’ve set up payment method in Mailgun. And this is my error log

    [2019-12-09 16:24:16] WARN

    NAME: InternalServerError
    MESSAGE: The server has encountered an error.

    level: normal

    "The email service was unable to send an email batch."
    InternalServerError: The server has encountered an error.
        at new GhostError (/var/www/ghost/versions/3.1.1/core/server/lib/common/errors.js:10:26)
        at /var/www/ghost/versions/3.1.1/core/server/services/bulk-email/index.js:103:45
        at Request.callback (/var/www/ghost/versions/3.1.1/node_modules/async/dist/async.js:4617:26)
        at IncomingMessage.<anonymous> (/var/www/ghost/versions/3.1.1/node_modules/mailgun-js/lib/request.js:331:19)
        at IncomingMessage.emit (events.js:215:7)
        at IncomingMessage.EventEmitter.emit (domain.js:476:20)
        at endReadableNT (_stream_readable.js:1183:12)
        at processTicksAndRejections (internal/process/task_queues.js:80:21)

    Error: Too Many Recipients
        at IncomingMessage.<anonymous> (/var/www/ghost/versions/3.1.1/node_modules/mailgun-js/lib/request.js:327:17)
        at IncomingMessage.emit (events.js:215:7)
        at IncomingMessage.EventEmitter.emit (domain.js:476:20)
        at endReadableNT (_stream_readable.js:1183:12)
        at processTicksAndRejections (internal/process/task_queues.js:80:21)