Outlook 2010 says it sent emails, but only half get through

I have a desktop with Windows 7 Pro and Outlook 2010. Been using Outlook trouble free for years. But for the last few months some emails I thought had been sent were not received. I have several accounts coming into the inbox of the same Outlook data file. One account is a hosted exchange account, and items sent from it are reliably received. The other three accounts are Mediacom accounts from my ISP - POP accounts. I send most emails from one or another of the POP accounts, and most get through. But some don't. Yet all go into the "sent" folder as if all is OK. Using an unrelated yahoo email acct as a test recipient, I put it's inbox on one screen and my Outlook Inbox on the other. I can send five quick test emails to the Yahoo acct, and can watch them appear in the Yahoo inbox. But if I send five of them, only 2 or 3 appear. The others go into the bit bucket somewhere, with no errors or bounces to let me know. Thinking perhaps there is a problem with the outgoing Mediacom server (mail.mediacombb.net) I googled for people with similar complaints. None found. Must be me. But what?