Are you sending an email from a third party email client such as "Outlook"? Here is some information below that you may find useful.
Let me know if this helps.
"When you are using email, the Verizon web-based email application, you can send attachments that are up to 8 MB (megabytes) in size. Emails sent using 3rd party email programs like Outlook, Lotus Notes, or Eudora, however; can contain attachments up to 20 MB"
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