Introduction
If your server uses MailChannels to send mail, and an email account on your server has been marked by MailChannels as a spamming account, MailChannels must unblock the account once you have taken the appropriate steps to make sure the account is no longer sending spam.
Procedure
These steps, as well as the procedure to unblock accounts in MailChannels, are available in MailChannels' documentation here:
Fixing the 550 5.7.1 [ESA] sender blocked error
Note: MailChannels is third-party software, so you must contact them for any issues related to their service. Their general support page is here: