POP3 accounts under external software

Hi there.

I am facing some issues regarding my POP3 accounts under my CP. It seems that everything is working just fine from the Roundcube Webmail, I might say without any issue at all. But if I try to use any software like Outlook, Thunderbird or even a cell phone to get my emails under my accounts in the server, I can receive emails but I cannot send any email from my server.

I dont know if someone faced the same issue here, but I try to check in the forum for similar posts, somehow I did not found any regarding this issue.

I kindly ask if someone can help me on this. My best regards to you all.