Sending, receiving, and organizing email in Outlook.com
Hi, let me help you.
Please check if your browser window or the compose pop-out is just too narrow. Outlook Web sometimes collapses the Send button into a "..." more-options menu or hides it entirely when the compose window is small, especially if you're composing in a pop-out window rather than full screen. Try maximizing the compose window or the whole browser, and check your browser zoom level (Ctrl and 0 to reset it to 100%), since a zoomed-in view can push the button off-screen.