It is simple really. the author of the add-on is suggesting you merge to the outbox so you can see your results before the final send. There is not get it back if you merge goes funny for some reason and say you send the wrong name to the wrong address or such.

'Mail Merge' is an addon extension program written by Alexander Bergmann that can be used at your risk. It is not part of Thunderbird, but it is designed to work with Thunderbird.

re :Use "Send Now" as "Deliver Mode" at your own risk! That is not an error message. It is an advisory message which Mail Merge has popped open because you have chosen to 'Send Now'.

It is advising you to use 'Send Later', so emails are put into 'Outbox' for checking prior to sending. After all, you would not want the wrong data going to the wrong person. So 'Send Later' offers a means of checking the results prior to sending.

But why would you not want to check and see what you are sending before sending? I would have thought most people would want to be very clear about what is being sent and to whom. There are only a few very exceptional cases where emails can be recalled.

From a business point of view, knowing exactly what is being sent and to whom is important as errors could be embarrassing at the very least or catastrophic in the worst case.

If you do not want to ever check what you are sending then you must accept the risk.

re :Please let me know about this. If something goes wrong then usually you are are first to know. There is no way of knowing what special circumstances are going on on your computer or the data you are using. The owner of the addon cannot test for all cases where someone has used different combinations of various addons. If you do find something really weird is occuring then the first action would be to disable the various addons to find out what is causing the conflict. If Mail Merge works after disabling other addons then clearly there is nothing wrong with Mail Merge. Only you can then work through switching addons back on to see where the issue lies and then report that problem to the owner of the addon.

Hence why you are being advised to use 'Send Later' or accept the risks.