TL;DR short version
Tags (or labels, or flags, or whatever you mail client calls them) can be stored on the server, but the IMAP specification defines it as optional. For Thunderbir, if the server doesn't allow tags, it will store them on a local file.
How tags are defined on IMAP
Message State Information:
Through the use of flags defined in the IMAP4 protocol, clients can
keep track of message state: for example, whether or not the message
has been read, replied to, or deleted.
These flags are stored on the
server, so different clients accessing the same mailbox at different
times can detect state changes made by other clients. (...) The IMAP4
protocol supports both pre-defined system flags and client-defined
keywords. System flags indicate state information such as whether a
message has been read.
Keywords, which are not supported by all IMAP
servers, allow messages to be given one or more tags whose meaning is
up to the client. IMAP keywords should not be confused with
proprietary labels of web-based e-mail services which are sometimes
translated into IMAP folders by the corresponding proprietary servers.
How Thunderbird handles IMAP tags
Thunderbird tries to store tags on the server using IMAP keywords. If
the IMAP server doesn't support that it will store lags locally in the
.msf file for the folder. That means that another PC can not see the
A quick and dirty way to test where tags are stored would be to exit
Thunderbird and delete the .msf file, and see if the tags disappeared.
A more sophisticated way is to log into the IMAP account and to check
whether the PERMANENTFLAGS server response to the SELECT command
contains * (see RFC 3501).This page explains how to manually connect
to an IMAP server.
Thunderbird 1.5.x uses Labels instead of tags but stores them in the
For sharing tags with another PC (or Thunderbird using a different
profile) you need to have defined the same tags to see them.
Thunderbird appears to have hard coded support for five tags that
mimic the old labels. If you use one of those tags on another PC and
delete those five tags on your PC you can still see the tags in the
remote folder though they will not be colored. It will display the
tags in the folder listing but won't list them in the expanded header
when you read the message.