Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
(I regret that I am not able to offer Secure Remote Assistance service at this time; However, your request is still open to other Experts in this category, and one of them may pick up on it at any time.)
The crucial part of that error message that you quoted, is the "554 message not allowed".
That may mean, that your email has violated policy in one of these respects (for instance):
- objectionable links (e.g. links that pretend to be related to one web server, but point to another)
- malicious attached files (e.g. one that Yahoo! Mail's scanners have detected malware within)
- bad header information (misleading header information is typical of spammers and other shady email senders.)
I suspect and I recommend, that you scan your attached file (and your whole computer) with a good-quality anti-virus scanner, to find if there might be a violation in the second sense listed above, of detectable malware infecting your files or your attached file(s).