Why is mail not getting downloaded in desktop client?

If mail are showing on webmail but not able to POP in desktop client can be mainly dependant on following

1. Configuration of Desktop Client

Reason: Under this mail will never be downloaded as some parameters in the detop client may have wrong values
Action: Check All the parameters in deskto clients to see that all values for paramters are generated with correct values

2. Local ISP Issue

Reason: Since mail is bein shown on the webmail interface, When mail are downloaded from server to local desktop client mainly its the local ISP same in case of send mails
Same can be in case of SENT mails..mail from dektop has to reach server and then despathced to the reciever mail id, mail can show in sent folder of the OUTLOOK but in case mail does not reaches server it cannot be sent despite at times if can show in sent folder of the mail cleint.

To verify perform the following

Login to webmail, if mail exist in your inbox but not showing in desktop client (Outlook)  means not able to download (ISP is used for download)
reasons ( mismatch of some coniguration of CLIENT or Issue with client.) And if this happens randomly then its mainly ISP issue

to further cross check if you any alternate ISP see if same issue occurs for both( should not)