It appears to me the SMTP-server for your mailaccount is blacklisted and sending is prohibited. Did you visit the link https://csi.cloudmark.com/en/reset?ip=172.58.29.217 which was provided in the error message? Otherwise, you should contact your e-mail provider and let them investigate and fix this.

This is not a Thunderbird error.

A blocked Email is one that is being temporarily rejected by the receiving server not because of any issue with the recipient's email address but because of the content that is contained in the message. In other words, it is the rejection of the message sent by you rather than any issues at the receiver's end

Regards, Will

Thank you for your reply. But it does not solve the problem. The contents is very personal , containing positive feedback, nothing political or offensive. However if I change to another email provider, it works. So the problem may stem either from the black list or other problem of the provider. Somehow the problem of errors message has stopped with the change of email account. Regards, tnguyen893