I can recieve email but not send it

I have set up my office email on eM Client, I have no problem receiving messages and I can send emails to email addresses at my company that use the same domain name but I cannot get an email to send to any other email address. I have tried to shut off the antivirus and firewall but neither worked. I have also ran the diagnostics on eM Client and both get the green check mark.I have looked through other questions on the forum but I haven’ t really seen one for this problem yet. I am just at a loss, I’m hoping someone will be able to help me out. Thank you!

Is your email server an internal one?  Have you checked with the administrator?  They may have restrictions on third-party clients send outside recipients.