I can’t tell you why this is doing this, but I can recommend some troubleshooting steps to try. It would be helpful if you had included more details. You did not say whether you are connecting to an exchange server, but if you are please tell us what version and patch level. If not, are you downloading to a pst? If a PST, the first least invasive thing to try is running scanpst against it to correct any corruption. If that does not resolve, remove your profile, create a new one and migrate the mail into the new one. If not, are you using cached mode? If you are delete the .ost file and let outlook create a new one. Certainly try running a repair on Office. If that does not resolve, try an uninstall and reinstall. Knowing the details helps when trying to recommend solutions. Please provide more details. Thanks.