I am running a mailing list with Mailman which uses Postfix to send the emails through Mailgun. I'm on Mailgun's (paid) Flex plan with a verified (non-sandbox) account.
The mailing list has about 30 members and the traffic is low. Still, when an email is sent, the log (/var/log/mail.log) contains entries with Domain ... is not allowed to send large batches yet.
postfix/smtp[33682]: 019053E815: to=<...>, relay=smtp.mailgun.org[44.231.238.210]:587,
delay=1181, delays=1179/0.02/2/0.13, dsn=4.0.0, status=deferred (host smtp.mailgun.org[44.231.238.210]
said: 421 Domain ... is not allowed to send large batches yet (in reply to DATA command))
I was unable to find any information about this error message. What's a good workaround for this issue – should the outgoing emails be throttled or should I request an increase of the batch sizes from Mailgun?
The solution was to contact Mailgun support. They raised the limits on my account and after that, the emails got delivered – the mail.log file now has status=sent (250 Great success) entries.