I am getting the same error.
Error Log File:
{"name":"Log","hostname":"ghost-ubuntu-s-1vcpu-1gb-fra1-01","pid":1223,"level":50,"req":{"meta":{"requestId":"039e7b2b-7b9a-4930-b9af-b6726da55ee0","userId":"1"},"url":"/email_preview/posts/5f78e04a5abbb6641544e095/","method":"POST","originalUrl":"/ghost/api/v3/admin/email_preview/posts/5f78e04a5abbb6641544e095/","params":{},"headers":{"x-forwarded-for":"178.233.17.38","x-forwarded-proto":"https","x-real-ip":"censored","host":"censored","connection":"close","content-length":"35","accept":"application/json, text/javascript, */*; q=0.01","x-requested-with":"XMLHttpRequest","x-ghost-version":"3.35","app-pragma":"no-cache","user-agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.121 Safari/537.36","content-type":"application/json; charset=UTF-8","origin":"https://censored","sec-fetch-site":"same-origin","sec-fetch-mode":"cors","sec-fetch-dest":"empty","referer":"https://censored/ghost/","accept-encoding":"gzip, deflate, br","accept-language":"tr-TR,tr;q=0.9,en-US;q=0.8,en;q=0.7,de;q=0.6,vi;q=0.5,th;q=0.4,nl;q=0.3","cookie":"**REDACTED**"},"query":{}},"res":{"_headers":{"x-powered-by":"Express","cache-control":"no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0","access-control-allow-origin":"https://censored","vary":"Origin, Accept-Encoding","content-type":"application/json; charset=utf-8","content-length":"267","etag":"W/\"10b-h600eyTqStHjNUAD25TfzdPyWoo\""},"statusCode":500,"responseTime":"554ms"},"err":{"id":"e1e97f70-07a0-11eb-a938-6b95e7d04462","domain":"https://censored","code":"BULK_EMAIL_SEND_FAILED","name":"EmailError","statusCode":500,"level":"normal","message":"The server has encountered an error.","context":"\"The email service was unable to send an email batch.\"","stack":"EmailError: The server has encountered an error.
at EmailError.GhostError (/var/www/ghost/versions/3.35.3/node_modules/@tryghost/errors/lib/errors.js:10:26)
at new EmailError (/var/www/ghost/versions/3.35.3/node_modules/@tryghost/errors/lib/errors.js:34:20)
at /var/www/ghost/versions/3.35.3/core/server/services/bulk-email/index.js:236:30
at tryCatcher (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/util.js:16:23)
at Promise._settlePromiseFromHandler (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/promise.js:547:31)
at Promise._settlePromise (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/promise.js:604:18)
at Promise._settlePromise0 (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/promise.js:649:10)
at Promise._settlePromises (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/promise.js:725:18)
at _drainQueueStep (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/async.js:93:12)
at _drainQueue (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/async.js:86:9)
at Async._drainQueues (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/async.js:102:5)
at Immediate.Async.drainQueues [as _onImmediate] (/var/www/ghost/versions/3.35.3/node_modules/bluebird/js/release/async.js:15:14) at processImmediate (internal/timers.js:456:21)
at process.topLevelDomainCallback (domain.js:137:15)
Error: Forbidden
at IncomingMessage.<anonymous> (/var/www/ghost/versions/3.35.3/node_modules/mailgun-js/lib/request.js:327:17)
at IncomingMessage.emit (events.js:327:22)
at IncomingMessage.EventEmitter.emit (domain.js:482:12)
at endReadableNT (_stream_readable.js:1221:12)
at processTicksAndRejections (internal/process/task_queues.js:84:21)"},"msg":"The server has encountered an error.","time":"2020-10-06T06:55:13.143Z","v":0}