Rated 4 out of 5 stars
I tested with SeaMonkey German 2.39 final Mozilla/5.0 (Windows NT 6.1; WOW64; rv:42.0 from official download area) Gecko/20100101 Firefox/42.0 Build 20151103191810 (Classic Theme) on German WIN7 64bit:
Works fine, and the "mozillazine"-workaround heals the problemwith the "eaten" Disposition-Notification (see http://blog.kamens.us/send-later#caveats). The other workaround does not work for me, neither with SM 2.39 nor with TB 46.0a1 (2015-12-30)
Apparently, adding Disposition-Notification-To as described in the user guide is broken in the current version of Thunderbird. This is due to a Thunderbird issue, not an issue with Send Later. See:
Rated 5 out of 5 stars
Often 1/10 second after I have sent a message I find out that I forgot something.
Latest Version works fine with SeaMonkey 2.35 ... SeaMonkey en-US 2.39a1 Mozilla/5.0 (Windows NT 6.1; WOW64; rv:42.0 from official download area) Gecko/20100101 Firefox/42.0 Build 20150801224600 (Classic Theme) on German WIN7 64bit