OpenSSL does not like .EML files to end with a . (which SMTP needs to process it). See https://gist.github.com/anonymous/7233372 and https://gist.github.com/anonymous/7233329 The former throws this error, the latter not. This is not the width of the base64 encoding (which I initially thought) but the terminating period.
|Error reading S/MIME message|
|6900:error:0D06B08E:asn1 encoding routines:ASN1_D2I_READ_BIO:not enough data:.\crypto\asn1\a_d2i_fp.c:251:|
|6900:error:0D0D106E:asn1 encoding routines:B64_READ_ASN1:decode error:.\crypto\asn1\asn_mime.c:193:|
|6900:error:0D0D40CB:asn1 encoding routines:SMIME_read_ASN1:asn1 parse error:.\crypto\asn1\asn_mime.c:528:|