[Exim] Cannot send base64 attachments

I don't know if this is an exim, mutt, or Linux thing.. I've searched
for answers but have not been able to find any.

My system is Debian, using Mutt 1.3.28, Exim 4.10. I cannot send an
attachment using base64 encoding. If I set the attachment encoding to
quoted-printable in Mutt, it will go. I can send base64 encoded
attachments from Windows, so it's not, as I first suspected, my ISP,
unless it's barfing at something in the format sent by my Linux system.
I'm sending to a smarthost.

What happens if I try to send a base64 attachment is that it begins to
send, or begins a queue run, and then finally times out.. The file I try
to send is about 20K.. sometimes the mainlog shows that some 16K are
sent and other times it shows the whole file being sent but then
apparently never receives the final acknowledgement from the server.

I'm wondering if this is some setting which I'm overlooking or if it's
something with either Mutt or Exim.

It seems that in the past, after failing to successfully send a
tar-gzipped file that after splitting it up and sending separate smaller
attachments, that I've been able to send. I'd thought that there might
be a size limit imposed by my ISP, but, since Windows works, and a
"quoted-printable" attachment of twice the size goes throught
successfully, that this is not the case.

Thanks to anyone who might be able to help with this.. it may have
nothing to do with exim, but I thought I'd start here..