If you can't send mail, but you can receive, and you're using the Mac Mail app, the cause could be malware. Here's the simple fix.
Can’t send mail from Mac Mail app? Here’s a fix.
How to get rid of “HPDeviceMonitoring.framework will damage your computer” pop-ups on your Mac
Here's how to fix the pop-up messages saying "HPDeviceManager.framework will damage your computer" in three easy steps. (HP's method takes 12 steps.)
Best Mac Anti-Virus Package: Intego Internet Security X9
Intego's Internet Security X9 is the best Mac anti-virus package you can buy. Includes network protection. Unobtrusive, yet effective.