You can receive emails but cannot send
Many major internet service provides (ISP's) may block outgoing mail server ports to prevent their services being used to send spam emails. The most common port that are blocked is port 25, but for non-SSL settings we do recommend that you use port 26. You should review your email account details (in Outlook / Mac Mail etc.) and adjust if necessary.
Using Port 26 but this still does not work
Some ISP's may block port 26 as well as port 25. When this happens, you may wish to contact your ISP and discuss whether they can enable a port for you or alternatively receive instructions on what SMTP port or servers you can use that will work through their network. Most commonly, this information will be able in your ISP's FAQ section on their website.