The reason behind this issue isn’t clear but it is related to the mail server. The error message that appears in an x.x.x format (like 4.3.4) is an error message originated in the mail servers. So the problem is caused by something that only the mail provider will know. This issue is usually common among the Century Link mail provider users.
Method 1: Contact your Mail Provider
Since the problem is from your mail provider’s end, there is not a lot that you can do here. One of the things that you can, and should, do is to contact your mail provider and tell them about this issue. Your mail provider might have an issue or they might work and fix this for you.
Method 2: Edit as new message
This isn’t a solution but more of a workaround and it doesn’t always work. So, if you really want to send an important email and you keep seeing this error message then follow the steps below to check whether it resolves the issue or not.
Method 3: Sign in to Webmail Server
Again, this isn’t a permanent solution and doesn’t work for everyone but it should resolve your issue till your mail providers provides a permanent fix. Here are the steps you need to follow to fix this issue
Method 4: Delete Emails from Webmail Server
Note: This solution is specifically for Century Link users but you can try it as well if you have a different mail provider. This issue might be caused by undeleted emails on your Webmail server as well. Sometimes, when you clear the messages from your ThunderBird, the Century Link doesn’t delete those messages from their servers. So, you keep seeing that the messages are deleted but they are only deleted on the ThunderBird and not the actual server. So, simply going to the Century Link Web Mail and deleting the old messages usually resolves the issue for a lot of users. Once done, simply come back to ThunderBird and try to send the messages again. It should work fine now.
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