I receive my emails via Outlook... and have doing

I receive my emails via Outlook... and have doing so for a year. Has always worked ok, but now I am using Comcast in Fl (was using Mediacom in Iowa) and now I can't get emails to send (just sit in my outbox). Also, a queston about POP vs IMAP.... I have a new HP laptop and now am using Outlook 2007. My old computer let me put follow-up flags (with dates) on emails. My new computer with Outlook 2007 will only let me flag an email, I can't add a follow-up dates. Someone told me I should change my mail from IMAP to POP. What is the difference... is one better than the other???