The 30 day expiration is for player-sent mail

The 30 day expiration is for player-sent mail. Many of the Blizzard official pieces of mail will have a longer duration (or no expiration at all).
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For example, I received a response to a support ticket in the mail. The expiration was 365 days. When I boosted a character to 90, all gear and bags were replaced with the level 90 items -- and the previously equipped items mailed back. The expiration time for that mail was 180 days.

It's possible that the Hearthsteed mail has no expiration at all. Or a really long one.