Is your corespondent on AOL? Unless they changed their spam filters, your domain may be blacklisted. It appened to me several times. They where filtering on the sender's address and not on the content... If only one spam is received with a forged sender address from a given domain, then that domain could get blocked.
Ask your corespondent to check their Spam or Junk folders.
Try re-sending the message with a different subject line.
If there are any attachment, rename those with a different extention, like .txt and add renaming instructions in your message. Some e-mail systems are set to suppress messages containing some attachemnts to help "protect" their clients from malicious content.
Check if your message contain some sensitive words, like anything with a possible sexual slant. If that's the case, try rephrasing your text.
Electro, April 2014