''re :mail server responded: Reject for policy reason.'' This is not caused by Thunderbird per se. I'll try to cover most of the usual things that might cause that response from server. That is a message from the server and it often (but not exclusively) means the email has been blocked because it's violating a server spam policy. Servers have their own Spam Filters and it may be you have something in the email they do not like. The problem server maybe the server you are using to send email or the server of the recipient. But I would start assuming it's your smtp server. Check the email content: In email, if there are a lot of links or remote content in images (frequently a problem when people copy an image from a webpage and do not realise that image is more than just an image or if they try to Reply or Forward an email that contain links etc). Sometimes it's the use of particular words, lots of colourful text and words using alot of Capital letters. Sometimes, it's due to a signature in the email which contains a link and or image with link. Sometimes it's due to the type of Attachment. Or if you have been sending a lot of bulk mail emails. I have known some servers to reject due to policy because they would like you to logon to webmail and reset a password if it's been in use for a long time. Then you would need to update Thunderbird saved passwords with the new password set in webmail. Do check computer to see if it's using a VPN because many servers do not like VPN as they hide your IP, so servers start to block as they think you might be a hacker. I know many Anti-Virus products now offer VPN and in some cases it has been auto enabled. You need to switch VPN off or create a split tunneling and set up it up so Thunderbird uses it. Do check to see if you have an Anti-Virus product scanning outgoing mail. I have known some Anti-Virus products to add their own data to the end of an email and server may not like what it contains eg: links etc. If you use an Anti-Virus to scan outgoing mail there is usually a setting to stop them adding content. Also check to see if you are sending emails in Both Plain Text and HTML. I've known some servers get twitchy if both are not included. * Settings > Composition * 'Sending Format' * Select 'Both HTML and Plain Text'