There are usually a few reasons why one can't send emails but can still receive them.
1. The SMTP settings are incorrect in your account details.
2. You're trying to send via a different account than you are connected to e.g if you have two or more accounts set up with different email providers.
3. The provider's SMTP server is playing up or temporarily down.
Number 3 happens lots of times but the provider, for some reason, doesn't pass the info on. Check out their website forums for further info, see if it's happening to other users.