MailMarshal Temporarily Rejecting Email
MailMarshal Temporarily Rejecting Email – When sending email to a MailMarshal server, the server responds with a the following error:
‘421 SMTP service is temporarily unavailable’
This is a temporary rejection of email by the MailMarshal server and email will start to queue up for recipient on the sending mail server.
This is a result of DOS (Denial of Service) and DHA (Directory Harvest Attack) attack prevention settings on the MailMarshal server. If a lot of email is being sent to the recipient from a single IP address, the MailMarshal server believes this to be a potential attack and will temporarily reject the email.
To prevent this, the recipient will have to whitelist the IP addresses of the sending mail servers in their MailMarshal server.