Did your phone recently get a 4.4 update?
Is the attachment download folder (in AquaMail) set to a "real" microSd card?
If so, please reset the download folder to the default location, "downloads" on the "large built-in storage".
In 4.4, Google severely limited applications' access to the "real" memory card.
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If you're not using a "real" memory card, please check if there is room left on the "large built-in storage".
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And if that's not it, please try the latest 1.4 build from this forum (under "development builds")