Weird email alert content when using mailing list address

[juste]

Hi,

My cacti and thold have been working fine for a month until i replaced the alert email addresses with a mailing list recently. Now, all alerts' content recevied through the mailling list will become attachments (1 plaintext, 1 html and 1 PNG attachment). But it works fine again if i switch back to multiple addresses, i.e. a@abc.com, b@abc.com, ...etc.
My cacti use a smtp direct connection to an exchange server for sending emails and i am using Cacti 0.8.6j, Plugin Archi 1.1 and thold 0.3.4.
Any ideas?

Regards,
Juste