We recently noticed a problem with the “Mail” app on Android phones failing to send mail when the phone has an IPv6 connection (either via Wi-Fi or 4G LTE). The problem was traced to the Mail app sending an improperly formatted IPv6 literal in the EHLO command. We’ve updated our outbound mail service to account for this discrepancy.
RFC2821 defines and IPv6 address literals as such:
address-literal = "[" IPv4-address-literal / IPv6-address-literal / General-address-literal "]" IPv6-address-literal = "IPv6:" IPv6-addr
The Android Mail app is omitting the “IPv6:” string in its literals.