Getting Mailer Daemon's here

[NeptuneMidheaven at aol.com]

Just testing here.... I have tried to post to this list twice and keep 
getting mailer daemons!

"Your message cannot be posted.
 It has the content-type: multipart/alternative,
 and this list accepts text only"

How can I stop this from happening??  I am sending this email from my MAC.
The other ones came from  my PC where all I did was copy part of Tom's reply 
to me and answer him.  I only use AOL mail.
Val