Get rid of Outlook and switch to a webmail service like everyone else is doing. Your mail will never again go missing.
Outlook is ancient history.
Webmail is not without problems too. If you lose your internet connection you don’t have your email. Outlook is nice to have a local copy.
That being said, you could have a corrupt profile in outlook or your provider(if it is Exchange) could have upgraded to a newer version that is having issues with Outlook 2007. I would certainly suggest that you upgrade to Outlook 2016.
Actually no, Outlook is extremely popular especially with businesses. But, yes logging into your provider’s webmail portal will verify you user and password hasn’t been compromised or service interrupted.
Thunderbird is a pretty decent option.