I had the same problem and fixed it thus.
System Preferences Encryption and Keyrings PGP Passphrases
Then set Remember passphrase for xxx minutes and this, in my case,
resolved the problem.
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Remembering passphrase does not work
https://bugs.launchpad.net/bugs/174640
You received this bug
Public bug reported:
Binary package hint: thunderbird
Ubuntu 8.04, Thunderbird 2.0.0.14, Enigmail 0.95.6
This only started occurring after upgrading to 8.04. I have to re-enter,
by typing, my private key passphrase for each individual email. Under
passphrase setting on Enigmail I have it set
** Attachment added: Dependencies.txt
http://launchpadlibrarian.net/14552608/Dependencies.txt
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openpgp have to re-enter passphrase for each email
https://bugs.launchpad.net/bugs/230976
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