Rated 5 out of 5 stars

This is a great add-on, that corrects one of Thunderbird's most evident flaws in the "Compose" window. I've wished for a long time for something like this (which should have been Thunderbird's default, actually) and now I can finally manage recipients for my messages in a much more efficient way. I would have rated this add-on with five stars if it weren't for a small bug that I've just come across. Hopefully, it can be corrected by the developer. The symptom is this: every time I try to send a message, for some reason Thunderbird complains that "There is no TO recipient, only CC/BCC". This is obviously not the case, as I can see that there is indeed a TO recipient. And if I ignore the warning and "send it anyway", the message is sent without issues. I don't know what is triggering this warning. I'm running Thunderbird 17.0.8 (locale es-AR) on Windows XP (Windows also in Spanish). By the way, the warning text is displayed in English, which is neither my Thunderbird's nor my Windows' language, so I'm guessing the warning is caused by this (or maybe another?) add-on

(UPDATED with bug symptoms and system information)

BTW, I just tested the bug described in the comment below mine (the bug with the attachments) and I can't reproduce it. I'm not seeing the attachment being incorrectly added to message #2

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UPDATE 2:

Eureka! I've found the cause of the bug that I was experiencing. This add-on is found "Not guilty" and therefore its rating increased to some well-deserved 5 stars :)
The "guilty" add-on is Flexible Identity (https://addons.mozilla.org/en-US/thunderbird/addon/flexible-identity/). It has an option that displays a warning "if there is no TO (only CC/BCC) recipients". Unfortunately, it seems that these two add-ons (MRC Compose and Flexible Identity) are mutually incompatible. Flexible Identity simply doesn't work when MRC Compose is being used :(
I hope this incompatibility can be fixed, because I'd hate having to choose between one or the other.

This review is for a previous version of the add-on (1.5). 

Thanks for your feedback !
I understand how annoying is what you describe, but I just tested your test case and had no problem replying.
Can you list other addons installed in your Thunderbird ?

And thanks for testing attachments too !