Some of the popular e-mail providers (Google, Yahoo, etc.) scan for terms like "USPS" as part of their spam filter. Because of this, some of your confirmation e-mails may be placed in the spam folder of your customers.
As always, teaching your mail program that these e-mails are not spam (through filters or the "This is not spam" button) will help for future messages, but it won't necessarily help your customers get their first message.
If you are experiencing this issue, we recommend that you change your current wording like you see below. This should bypass what Gmail, Yahoo Mail, Windows Live Mail, etc. are recognizing as "spam" words and thus the e-mails should successfully reach your customers' inboxes.
Head to your Account Settings (the gear icon in the upper right).
Choose Shipment Notification or Delivery Notification, depending on the confirmation email you'd like to edit.
Select Edit under "Actions" for your template. Edit the following:
We wanted to let you know that your order (#[Order #]) was shipped via [Carrier Name], [Shipping Service] on [Ship Date]
Example Corrected Sentence:
We wanted to let you know that your order (#[Order #]) was shipped on [Ship Date] and can be tracked with the tracking number below.
Here's what the e-mail editor would look like if you use our example sentence: