I have on several occassions failed to include a period of the right name. I have used ".org" instead of ".com" stuff like that. You might want to check you email settings on your computer or the service you use to send your emails.
I keep getting a " sending failed" when I sent out an email. ( I can receive emails but can't reply back to them ) Any ideas?
Sometimes without notification, your email provider may change something so that the email port number has to change if you are using a mail program (Like Outlook) to open you email.
If it is web mail, it could be anything from heavy traffic, incorrect email address, correct email address but user deleted or discontinued their account, Maintanance on either your sending server or their receiving server for mail.
I dont know.