[unikernel] signer, submission, spamfilter, verifier: Unused variable postmaster

[hannesm]

I get:

File "unikernel.ml", line 101, characters 8-18:
101 |     let postmaster =
              ^^^^^^^^^^
Warning 26 [unused-var]: unused variable postmaster.

when compiling unikernels. I'm not entirely sure whether this should be used somewhere?

[dinosaure]

Yes, this value exists because, initially, I mainly copy/paste the way to launch an SMTP server (all unikernels are an SMTP server in the end). I will clean-up that 👍.