[exim] Verifying correctness of headers_add?

Recently I discovered that one line of my config file had read
headers_add = X-DCL R default
instead of
headers_add = X-DCL: R default
for a while. I hadn't noticed this until mails generated by reportbug
were rejected by bugs.debian.org because of the malformed header. [1]

Is there any way to run a sanity check on the contents of headers_add
options? (exim -bV doesn't check them.)

Thanks,
Adam

[1] Yes, this question arises from a Debian system, but AFAIK there's
nothing Debian-specific about it.