This happens when your libc supports IPv6 addresses, but your kernel does not. When Qremote tries to open an IPv6 socket to send the mail the syscall fails. Recompile with -DIPV4ONLY.
If this does not solve your problem completely and you still get this error on some specific domains read on.
AOL uses a multiline opening in the SMTP connection. It's not absolutely clear if this is a violation of RfC 2821, but it's at least suspicios. Apply patches/AOL-greeting.diff and rebuild, then you can send mail to them again.
Control messages like subscription confirmation messages of mailing lists run by ezmlm bounce with "domain of mail address is syntactically incorrect". This happens because ezmlm breaks the length limit for localparts. RfC 822 and 2822 both tell a localpart is not allowed to exceed 64 characters. Apply patches/localpart-len.diff to allow localparts up to 256 characters. Note: this file is not included in version 0.8, you have to get it from the subversion repository.
Qsmtpd ignores most environment settings used by qmail-smtpd, including RELAYCLIENT. Use control/relayclients (IPv4) and control/relayclients6 (IPv6) to set the hosts and networks allowed for relaying. The program addipbl from tools directory will help you adding entries to this file. You can build it with "make tools/addipbl".