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This + color-tagging the labelled mails would be outright fantastic! For example Fastmail is offering the same concept: https://www.fastmail.com/blog/fastmail-labels/
There's maybe a way to automatically create and apply the necessary tags, to mimick Gmail/Fastmail's Labels behaviour..! Could you please generalise the Labels add-on for other accounts too?

Questa recensione riguarda una versione precedente (0.2) del componente aggiuntivo. 

Thanks for your review! This extension does not allow adding own tags, it only reflects the tags already set in Gmail.

There is a generic tagging functionality built into Thunderbird: https://support.mozilla.org/en-US/kb/message-tags, maybe you are looking for that?