When sending or receiving emails, the total size of the email cannot exceed 25mb uncompressed.
It should be noted that since the email server will add some information (mime) as well, and because the documents will be uncompressed before sending, this usually means that the total of all the attachments should not exceed 15-20mb. (PDF documents, for example, are compressed. When being sent, the server will uncompress the file which increases the actual size of the PDF document resulting in the possibly of exceeding the limit.)
If you send a file that exceeds the allowable limit, you will get a warning and you will not be able to send the email until you remove the file.
If a file is sent to you from an external source that exceeds the allowable limit, you will not receive the email and there will be no warning. Depending on the sender's server they might or might not receive a bounce-back message warning them that their message exceed the file size limit.
If someone says they sent you a file and you did not receive the email, the first thing to check is the file size.
Please note that you can share larger files using SharePoint or OneDrive.