WOULD BE NICE:
- switch to the MD5 and BASE64 implementations of gnulib
  (not necessary since everything works, but less code we have to take care
  of...)

MAYBE LATER:
- system configuration file $SYSCONFDIR/msmtprc 
  - overridden by user conffile
  - overridden by command line
  default: load /etc/msmtprc, then ~/.msmtprc
  --file just changes the user conffile, /etc/msmtprc still gets read

MAYBE LATER:
- the special command mx makes an account use the mx domain for each
  recipient. Option --mx, not usable with --host or --account.
  ONLY THESE SETTINGS ARE ALLOWED WITH MX:
  domain, from, keepbcc, logfile
  ==> no TLS, no DSN, no AUTH
