Not sure what you mean with opaque signed though...
There are two ways of signing
data. One way signs the data and puts a
blob underneath, leaving the data readable. The other way, opaque
signing, renders the data unreadable.
Some mail servers decide that the formatting of a signed message is not
correct, and they change it, rendering the signature invalid. Opaque
signing prevents that.
dagdag is just a two-character rotation of byebye.