Let's dissect the error message. An email address, such as: john_smith@example.com has two parts: ''john_smith'' is the "local part" - it is specific to the owner or user of that email address. The rules for what letters or symbols may be used in this are rather relaxed and are set by the operator of the server. ''example.com'' tells us what domain the email address is hosted by. This part has rather more stringent rules about permitted characters. For instance, it is not allowed to be case-sensitive, unlike the local part. Nor can it have optional . or + characters, unlike some services' local parts. Now, your error message means that there are some "exotic" characters in the local part. This will most likely happen if you're corresponding with someone whose natural language uses an alphabet other than the usual Latin one we use in the western world. It may also happen with unicode/utf-8 characters, though I see these in incoming messages so I believe they should not be a problem within Thunderbird. I am not sure whether that message comes from Thunderbird or an email server. If it's from Thunderbird then this is something that needs to be fixed and your immediate work-around might be to use an alternative email client, or use webmail via your browser. But as suggested above, I think this is unlikely. If the message is from a server then you might need to use a different smtp server to send your message. Again, webmail via a browser may work for you, or you could try any alternative email accounts (with other providers) that are available to you. And finally, does the correspondent have an alternative email address that uses only Latin characters and that you could use instead?