I sent an email to my friend the other day but he couldn't receive it. I tested several email providers to find out what the problem was. I came to know It only happened on Apple mail. Then I noticed there were some URLs (not hyperlinks) in the email message. So I deleted them one by one and send it again and again. The culprit was an URL named 'http://www.friendstranscripts.tk/'. I deleted the URL and strangely could send it. (I could send and receive the email with the URL without any problems by Gmail though) Can anyone explain this? (If you wanna see this yourself, try sending an email only included the URL, http://www.friends~~.) My mac is Mac Mini 2012 late and on OS X Yosemite 10.10.4.