Regardless of what email program you use, you have the ability to add files and send them to your contacts. Not all email services are the same however. Email providers set limits on the size and types of attachments that can be sent. The most common reason that attachments can't be sent is that the file size is too big. While one service may allow attachments up to 10MB, another may only allow attachments of 1-2MB.
Compress the attachment. The most common reason that an attachment won't send is that it is too big. These limits are set by whoever you use for email, whether it's an email account through your ISP or through an online provider like Yahoo or GMail. You should check with your email service provider to see what the limits are for attachments. This is often found on the Support or FAQ page of their website. Download a compression utility like WinRAR or WinZip (or any of the other freeware and shareware utilities) and use it to compress your file to a size that meets the requirements. If you are sending multiple attachments, try sending one at a time.
Scan for viruses. Some viruses infect your computer and prevent it from sending attachments. Use your antivirus software to do a deep scan of your computer. Make sure to download any virus definition updates before you scan to catch the newest viruses and malware.
Change the file extension. You can occasionally get around an email provider blocking certain file types by changing the name of the extension. When you change the name, it does nothing to the actual file type. In Windows Explorer, select the file you want to send. Right-click it and choose "Rename." If you are trying to send an .exe (executable file), you might try renaming it "file.ex_" instead. If you cannot see the file extensions, you will need to go to Tools and then Folder Options and change your settings to view them.
Share the file online instead. If you still cannot send the attachment, considering using an FTP server or a file sharing service to send it. Some online services allow you to send very large files, although the size allowed is often determined by the level of membership you have with the site. Free members may be limited to smaller files than paid members.