I sent an email which got rejected by the receiving end's amavis:

X-Amavis-Alert: BAD HEADER SECTION, Improper folded header field made up entirely of whitespace (char 09 hex): References: ...5.15.camel@bar.qux>\n\t\n\t <1467314203.[...]

The header does indeed look bad:

    In-Reply-To: <1467536130.28737.15.camel@bar.qux>
    References: <1466866500.12705.15.camel@bar.qux>
           
             <1467314203.8891.23.camel@foo.bar>
             <1467536130.28737.15.camel@bar.qux>
    Content-Type: multipart/signed; micalg="pgp-sha1"; protocol="application/pgp-signature";
            boundary="=-AjoRxr3sex+M9D6ikFVw"
    X-Mailer: Evolution 3.20.3-1ubuntu1~xenial1
    Mime-Version: 1.0


The check in Amavis seems to be:

     elsif ($t_empty && $curr_head =~ /^([ \t]+)(?=\n|\z)/gms) { $mid = $1; $msg1 ="Improper folded header field made up entirely of whitespace"; # note: using //g and pos to avoid deep recursion in regexp $minor_badh_category = max(4, $minor_badh_category);


This is extremely unfortunate.

I expected Evolution to generate a properly formatted email.

Could Evolution run sanity checks on emails it produces?

(Also: When replying to an email with such a header, evolution reproduces it which may also yield unexpected results)