Once you press the "Send" button, it doesn't matter which email client you use. You message is on its way and out of your control. It is at the mercy of your ISP and the various servers along the away between your computer and your ISP's mail server and between your ISP's mail server and your recipient's mail server. Between you and your recipient, the only control that you have is your choice of mail provider or ISP. Sometimes you have a choice; sometimes you don't. Just this past week, I emailed my colleague across the hall. I still don't know if he has received my message. We are both restricted to our firm's Exchange server. I have seen it take months for large messages to get to their destinations via AOL.
You said nothing about your ISP, your email accounts, or their capacity limits. Avoid sending large files via email. If you have a choice of ISP, investigate the reliability of your options. If you are using Exchange or IMAP, make sure that you dson't exceed your capacity allocation. This is particularly important if you keep your mail on a POP3 server.