I had that problem with yahoo and there was often no explanation or reason for it until I was notified by a large advertising site that their site was experiencing a 50/50 ability for yahoo email to receive / send email utilizing yahoo where censors where blocking particular use of words.

It could mean something or not but I found it easier to finally leave yahoo as  I were sometimes unable to receive important email.  

If sending a simple "hello" email and having very vanilla content will get your message though, you may be experiencing same yahoo "censorship".

Hope that helps