merge_feature does not update "To:" header

[agouaux]

The email personalization via merge_feature on only seems to impact the message body, not any of the headers. It would be great if merge_feature also updated the "To:" header. So, assuming verp_rate 100% also set, have the "To:" header be the specific recipient instead of the list address.

[racke]

At least an option for that would be useful.

[mitar]

I had this patch in the past, but now it does not work anymore (see also history of the patch): https://github.com/tozd/docker-sympa/blob/master/patches/3006-personalize_recipient.patch