Sony Xperia User Manual page 213

Tap message category
Export starts.
OK
Export is complete.
Exporting Message Data
Where Message Data is Exported
Exported message data is saved to the locations below in SD Card or handset memory.
Received message data: private/MYFOLDER/Mail/Inbox/Inbox.BCK
Sent message data: private/MYFOLDER/Mail/Sent Messages/Sent Messages.BCK
Draft/unsent message data: private/MYFOLDER/Mail/Drafts/Drafts.BCK
Filenames of Exported Data
Filenames of exported data are formatted as the year, month, and date saved to SD
Card/handset memory. For example, message data saved on June 1st, 2015 is
formatted as "15060100.vmg." Several files saved on the same day are formatted
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