Can send email through webmail but not through outlook..Spam?

I have a user at work trying to send an email to this law office but it bounces back when sending from outlook.
   
SMTP error from remote mail server after end of data:
    host mail.example.com [x.x.x.x]:
    554 5.6.0 Invalid content

The user can send the email through our webmail online and it sends fine. The smtp server it sends through online is the same as the smtp server setup in outlook.

Can the email be modified in such a way going through our isp that this problem occurs?
I do know we are on spamcannibals blacklist but from my understanding the ip would be blocked and if it comes from the smtp server through both methods (webmail or outlook)  id assume we wouldnt be able to send through either method.

I checked the log on our web host accounts email settings but i dont see any message pertaining to actual spam related info when this email was sent out.
ShotgunGaperAsked:
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smksaCommented:
from your local PC, try to telnet your mail server SMTP port, default is 25 but some hosting provider changed to other, need to check what exact outgoing port.

Hope it will help
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ShotgunGaperAuthor Commented:
I do get a response from the server using telnet. Its setup on port 26.
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