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very slow and opens multiple messages instead of the expected single reply message

Cette critique concerne une version précédente du module (1.0.4). 

Slow in IMAP, can't do anything for this. If there is multiple messages, it's probably because you have virtual folders or filters that copy your replying message in several places ! Further improvement could be an option to only display once a message with a specified ID, but the add-on can't invent which copy of the message you search !