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Apparently the phrase "Darn you autocomplete!" also applies to SMv16.
If you copy+paste a list of addresses into the TO: CC: BCC: fields when composing a new email in Smartermail it will attempt to autocomplete the list by trying to match what has been entered with a Contact and if you don't select a Contact it will blank out all the addresses you copied+pasted otherwise it will replace them with the Contact instead.
To duplicate try to paste the following into either the TO: CC: BCC: fields in a new email:
firstname.lastname@example.org; email@example.com; firstname.lastname@example.org; email@example.com
Autocomplete will attempt to match it with a Contact (or will display "No matches found") and if you TAB or ENTER or click into another field those addresses you pasted will disappear.
If typed manually the group of addresses works just fine.
Only solution I can find to workaround this is to create a new Contact Group each and every time you want to send an email TO or CC or BCC a group of people...which is fine if it is the same group of recipients you want to email regularly but if you are frequently doing this for different recipients it isn't a viable work around.
Tested on Chrome, Firefox, Edge, Safari, Vivaldi, and Brave with the same results on each.