Hello Carlos,
I dont think an upgrade will help, since the inspection is looking for non-RFC traffic and if the problem is that the ASA is seeing unsupported commands for SMTP, the same will happen on a different version.
Have you turned off the inspection to comfirm this is the problem?
You can tuned the inspection to allow the emails if you can find what the problem is:
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/inspect_basic.html#wp1224614
Enabling log for smtp might help to find the problem:
Ex:
policy-map type inspect esmtp advanced_esmtp_map
log
policy-map outside_policy
class inspection_default
inspect esmtp advanced_esmtp_map
service-policy outside_policy interface outside
Regards,
Felipe.
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