|if this refers to yahoo mail... I'd avoid the pay for help like the plague... |
Which email service are you using; and are you using it online directly, or via something like MS-Office/Outlook?
Also do you know the outbound (MTP) server for your email service?
Are you using Avast anti-virus? If so it is known to interfere with some emails outbound servers...
Have you tried to send with your email anti-virus monitoring disabled (albeit temporarily); similarly your windows firewall?