How to solve the issue where the POP3 services are running however the server is not accepting connections from external clients. In some cases, a telnet on 110 port starting from the local server can bring the POP banner, however the same process from any other server does not bring any banner.
After performing the basic checks like POP3 services are running, the administrator may face a bizarre issue where by using telnet utility we can see the POP3 banner from the local server but not from any other server, from all other servers the prompt will stay for a brief moments and then it will disconnect automatically, similar to the figure below.
telnet servername 110
This problem can be related to PopProxy component being disabled, the best way to validate if that is the root cause is running the following cmdlet, if it shows inactive on the PopProxy, then we have a line of action to solve the issue.
In order to bring that component as active the following cmdlet can be used
Set-ServerComponentState –identity <serverName> –Component popproxy –Requester HealthAPI –State Active
After that we can check again the components and at this time it should be Active, and if that is the case we can try to connect again on the POP3 service from a client.
After these changes, a simple test using telnet will result on the banner being displayed for all servers (not just the local server) as shown in the image below.