Since this morning, I cannot receive emails. I have tried disabling my firewall, but it doesn’t make any difference. I am working online, as I can send, but not receive. 
I cannot receive emails either
I cannot receive emails either
Sometimes when credentials are rejected, it is because you don’t have the correct settings for the server.
These are the correct combinations of port and security policy. The underlined ones are the best option for security if your server supports them. SMTP port 465 was reassigned and should no longer be used for SMTP, though many mail providers still recommend and use it.
SMTP
Port 587 = Force usage of SSL/TLS
Port 25 = Use SSL/TLS if available
Port 465 = Use SSL/TLS on special port (legacy)
IMAP
Port 993 = Use SSL/TLS on special port (legacy)
Port 143 = Use SSL/TLS if available
POP3
Port 995 = Use SSL/TLS on special port (legacy)
Port 110 = Use SSL/TLS if available
Another reason may be because your anti-virus or firewall is blocking the application or the port. You can disable the anti-virus and firewall and try again and see if there is any difference. If those applications are blocking eM Client, you will need to change their settings to allow eM Client to access the Internet.