The message threshold limit needs to be set to 0 in our configuration (otherwise we experience the BAD error described here: https://confluence.atlassian.com/display/JIRAKB/BAD+Command+Argument+Error+when+creating+issue+via+email+with+Microsoft+Exchange) I just upgraded JIRA from 6.4.1 to 6.4.2 and the threshold limit was reset to 5 which caused the error to return. I reported the same issue whenever a JIRA Service Desk upgrade was performed: https://jira.atlassian.com/browse/JSD-1747