Net::Smtp error

I’m using:

ruby 1.8.4 (2005-12-24) [i386-freebsd5]

I’m getting an error that I don’t understand when using Net::Smtp. This
code
works for most messages I’ve tried:

Net::SMTP.start(@smtp_server, @smtp_port, @helo_domain) do |smtp|
@users.each do |user|
smtp.send_message(mail.to_s, mail.from.to_s, user.email)
end
end

But fails with this error when some messages are sent (notably from
Outlook,
if that makes a difference):

/usr/local/bin/ruby18
/usr/local/lib/ruby/gems/1.8/gems/Rist-0.0.1/bin/receive.rb
/home/mark/lists/io-devel/". Command output:
/usr/local/lib/ruby/1.8/net/protocol.rb:244:in write_message_0': private methodsub’ called for 143740984:Fixnum (NoMethodError) from
/usr/local/lib/ruby/1.8/net/protocol.rb:300:in each_crlf_line' from /usr/local/lib/ruby/1.8/net/protocol.rb:310:inbuffer_filling’
from
/usr/local/lib/ruby/1.8/net/protocol.rb:308:in buffer_filling' from /usr/local/lib/ruby/1.8/net/protocol.rb:298:ineach_crlf_line’
from
/usr/local/lib/ruby/1.8/net/protocol.rb:243:in write_message_0' from /usr/local/lib/ruby/1.8/net/protocol.rb:258:inwrite_message’ from
/usr/local/lib/ruby/1.8/net/protocol.rb:287:in
using_each_crlf_line' from /usr/local/lib/ruby/1.8/net/protocol.rb:257:inwrite_message’ …
10
levels… from /usr/local/lib/ruby/1.8/net/smtp.rb:316:in start' from /usr/local/lib/ruby/gems/1.8/gems/Rist-0.0.1/bin/../lib/rist/list.rb:163 :insend_to_list’ from
/usr/local/lib/ruby/gems/1.8/gems/Rist-0.0.1/bin/…/lib/rist/list.rb:103
:in
`process_mail’ from
/usr/local/lib/ruby/gems/1.8/gems/Rist-0.0.1/bin/receive.rb:15

I don’t know if this is a bug or something I’m doing wrong. Can anyone
let me
know what’s happening?

Thanks a lot,

Mark