some recipients not receving my emails

I use Windows Live Mail 2012 and have gwi.net email .  I run a small business and communicate with my customers regularly.  Over the past few months more and more of my customers are not receiving my emails. I am having two issues: 

1. Generally on my end, it looks as though they have received it as the message appears in my SENT box.  However, they cannot find it, even in their spam or junk box.  If I sign into gwi's web based email, they DO receive the email.

2. And sometimes message are sent back to me with this error "host mx.tds.net[64.8.70.104] said: 554 5.7.1 [P4] Message
    blocked due to
spam content in the message. (in reply to end of DATA
    command)" 

but it is NOT to from the same people who's emails are just being lost in space.  SOMETIMES if I remove my signature the message will go through, but not always. 

Thanks for the help.