Can receive but not send messages in Zoho Mail

Can receive but not send messages in Zoho Mail

Hello! I was able to configure my email client successfully in that I can receive messages just fine. 

However, when I send messages out, they seem to go out fine (I don't receive any errors or anything), but the recipient does NOT receive those messages. I have tried sending messages from the web client, Mac Mail App and iOS Zoho Mail app - all with same result.

Could you help?