MFC r269550:
r269550 (by peter):

Check gethostname(2) return code - but even if it succeeds it may not
null terminate.

Temporarily use "From: $user@$hostname" rather than "From: $user".
The latter exposes incompatible behavior if using dma(8). sendmail(8)
(and other alternatives) canonify either form on submission (even
if masquerading), but dma will leak a non-compliant address to
the internet.