The largest file that you can attach to a message varies on how you send email.

For CubMail:

The largest message you can send is 10 megabytes (MB). However, files increase in size by about 30% when they are attached in email. Therefore, the files that you attach to an email in Cubmail cannot add up to more than 7.5 MB in size.

For all other email programs (such as Thunderbird, Apple Mail, etc.):

The largest message you can send is 30 MB. However, because files increase in size by about 30% when they are attached in email, the files attached to the email you are sending cannot add up to more than 21 MB.

Keep in mind that the text you send in the message also counts towards the message maximum. This means that if you are typing in a long message, the maximum attachment size is even smaller than what is listed above.

For receiving mail, the largest message that can be sent to you is 30 MB. This means that the largest attachment that can be sent to you is around 21 MB in size.