If MailGate is running in proxy mode on an IPv6 address, it is possible that IPv4 clients will be rejected. The reason is that MailGate cannot determine the original IPv4 address of the client, if there is a request from an IPv4 host, because it is only listening on IPv6.
The solution is to use IPv4-mapped IPv6 addresses inside of /etc/avira/avmailgate.acl:
Example for a single IPv4 host, e.g. 10.11.12.13
This also works for subnet ranges, e.g. 10.11.12.0/24