QuickBooks (or another program with the ability to get configured with your Simple Mail Transfer Protocol (SMTP) or Post Office Protocol (POP3) settings) is no longer is able to send e-mail messages with Symantec Endpoint Protection Cloud (SEPC) installed.
In some cases, e-mails fail to be sent and SEPC displays an error asking you to resend messages from Sent Messages.
Communications or compliance issues with the mail server.
Option 1: Contact the e-mail provider or application vendor to ensure full compliance with RFC 5321 (RSET in particular)
Option 2: Check for and resolve network connectivity issues. Packet captures may reveal that the connection was lost or terminated unexpectedly
Option 3: Change e-mail settings (SMTP server) to use an alternate port (If this is changed in an e-mail client, only incoming e-mail will be scanned)
Option 3 might give you the quickest and easiest resolution. If you have already tested turning off firewall, turned off application behavior monitoring, Intrusion Prevention, Browser Protection, Memory Exploit Mitigation (MEM), and nothing seems to resolve the issue, just find out from your e-mail service providers' documentation what alternate ports are able to be used, and then you can open QuickBooks, and put in the alternate port setting, and this should fix the issue, and allow you to run your firewall on "secure" with default firewall port settings.