If we're rejecting your e-mail, you should be receiving a reject code. Please see our list of reject codes for further reference.
We currently have our maximum message size set to 50 megabytes. Keep in mind that the encoding required to attach many files to an email adds a size overhead to your messages. This means that the effective maximum size of an attachment is really something smaller, often in the neighborhood of around 30 to 40 megabytes.
Even if you're within the size limit, email attachments are often not the most convenient or reliable way to send larger files. If you have any web or ftp hosting (XMission offers this to most of it's customers, as do many ISPs), you may consider uploading your file there and emailing the link. File transfer is implemented in many popular chat and instant messaging clients, and there are also services specifically for sending files to other people, such as You Send It.
When you're not using an XMission connection (from a hotel or coffeeshop, for example, or if you have another ISP), you need to provide us with a username and password when you send mail, using an option called "SMTP Authentication". You can generally find this option in the "Accounts" section of your email client's configuration, under "Outgoing Server" or "SMTP Server". Outlook calls the option "Outgoing Server Authentication", and Mozilla Thunderbird refers to it as "Use name and password". Make sure you use the same username and password that you use to receive mail.
If you're still having problems, it may be that the connection you are currently using is blocking the standard port (port 25) most mail clients use to send mail. This is often done to control spam. To get around this, you can tell you mail client to use port 587 instead of port 25.