Unable to write the email to the mailbox

From MozillaZine Knowledge Base

When downloading new mail, you may receive the following error message: “Unable to write the email to the mailbox. Make sure the file system allows you write privileges, and you have enough disk space to copy the mailbox”.

There are several reasons which are known to cause this error:

1) Insufficient disk space or insufficient privileges.
Most users have already ruled that out by the time they read this KB article. It wouldn't hurt to empty the trash and compact folders anyways.
2) Corrupt folder
Your mail folder could be corrupt due to not compacting it often enough. See compacting folders.
3) A problem with your anti-virus scanner.
Try disabling it. It can also happen with anti-malware software, though that is rarer. [1]
4) EncFS
This problem has been seen with every incoming email from multiple accounts - when the Mail folder resides within an EncFS-encrypted folder. Although this error is seen, a single email will be downloaded successfully. Email can be re-checked repeatedly, and the error just keeps popping up after each email. This was reproduced with Thunderbird (20070117) and EncFS 1.2.3. The workaround is to move the Mail folder into unencrypted space, and then create a symbolic link pointing to it. Then Thunderbird can check email properly again. -- Sy
5) There is a conflict between message storage formats.
An example would be a profile with a POP account that used mbox files that was later on reconfigured to use a global inbox which used maildir. This could happen due to changing the default storage type before creating the global inbox. [2]
6) The mbox files name did not match exactly the folders name.
Old versions of Thunderbird used "inbox" rather than "Inbox" as the folder name. This isn't an issue for Windows. However, if you are using Linux the mbox filename has to have the exact same case even though Thunderbird shows the contents of the file regardless of the case. [3]
7) an old bug
However, this bug has been marked as resolved for some time now.
This error can occur if Thunderbird downloads a message from a certain kind of POP server which contains a certain combination of control characters in the message body.
You can work around this bug by reading and deleting the message using webmail (so that Thunderbird won't see it as a new message), and then checking for new mail again. The only way to prevent it from happening is to use a POP server with better support for Thunderbird. If you are using your ISP as your email provider you might consider signing up for a free email account somewhere. This isn't meant to imply that there is anything wrong with your current POP server.

See also