Can someone kindly tell me what is wrong with zoho's smtp server?

Can someone kindly tell me what is wrong with zoho's smtp server?

For a long time I hesitated in using zoho because I could never send any email using email clients, I have tried it on Thunderbird, various android's mail client apps, as well as apps on iphone. All of then can receive emails and none of them can send emails with perfectly correct configuration. At this point I am going to conclude that the free personal account can only send emails from website only.

I haven't had this much hassle setting up inbox. I am just giving up in trying to get it working, it is just not possible. Time to move on to alternatives and give up privacy.