@guru Sorry I was talking about Dekko 2 (0.1.2, experimental release). I have the same thing with Dekko 1 (0.6.20, no longer supported) than you told. Both Dekko 1 & 2 versions are available in the Canonical image but only Dekko 2 with UBports. The undo button is not invasive as it appears and disappears (=adding then removing 1 row in the email list).
You can easily delete several emails at the same time by doing a long press in one of them. That will activate the "multiple selection page". After selecting the emails you can do some actions like deleting, mark as unread, etc. The problem would be the "several" word : how many emails can you select in order to successfully achieve the action ? I don't know but I think that my E5 will crash if I try with 40 in one time Hope that comment will help !