Error554 Message not allowed - 
After sending an email to Yahoo Email Account
------ Error Details ----------
Delivery to the following recipient failed permanently:
Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain yahoo.com by mta6.am0.yahoodns.net. [126.96.36.199].
The error that the other server returned was:
554 Message not allowed - 
There is no one solution to this problem, try all the following
#1 Yahoo Spam Filter BUG
I found that Yahoo may drop an incoming email to a user just because they suspect this email to be spam, the mail gets dropped silently and the user will never get it in SPAM folder and will not get any warning or notice
More information here:
#2 Your email server
Add masquerade to your /etc/mail/sendmail.mc
If you are using php mail() function, you may need to add "-f$fromaddress" as an argument
Also i wasn't able to send email from Thunderbird, i managed to fix it by changing Media mail server to "XXX.mail01.mtsvc.net" instead of "XXXX.accessdomain.com"
From Yahoo help article:
Indicates that your email was not accepted because the message in question may contain characteristics that Yahoo Mail will not accept for policy reasons.
- For example, it is against Yahoo Mail's policy to accept messages with malicious content or manipulated header information, such as:
A falsified origination point.
An altered sending date.
A forged email address.