Suppose we have an SMTP server listening on 127.0.0.1:25 and we compile our server with IPv6 enabled. Module zbxemail uses zbx_tcp_connect() to connect to the SMTP server and uses the first address "localhost" resolves to with getaddrinfo(). The problem is that the first address "localhost" resolves to (at least in my case) is an IPv6 address, so alert sending fails with "Cannot connect to SMTP server [localhost] [*** Cannot connect to [localhost]:25 [Connection refused]]".