Problem with sending mail using Zoho

Problem with sending mail using Zoho

Hi, I've been having problems with my Zoho mail personal account. I use outlook express to send emails to someone regularly but I recently sent an email with a jpg attachment of 17.5KB which didn't arrive and there were no error messages. I also tried using the web interface and this didn't work either. Whenever I try to send it again the recipient doesn't get it. Some emails arrive, some don't.The recipient is in the Whitelist on the Zoho mail web interface.