unexpected behavior of MX relay

I had set up my nameserver data as follows:

$ORIGIN twwells.com.
        IN      MX      10 twwells.com.
        IN      MX      20 admin.cyberenet.net.

I have installed exim on admin.cyberenet.net. When twwells.com
disconnects, instead of mail staying on admin and retrying for
twwells.com, it keeps getting resent to admin, whereupon it hits
the max hop count and bounces. Any ideas why?

I've appended my configuration.

#queue_only              = true
#preserve_message_logs   = true
collapse_source_routes  = true
errors_address          = admin@???
errors_copy             = admin@???
freeze_tell_mailmaster  = true
local_domains           = admin.cyberenet.net:news.cyberenet.net
log_received_recipients = true
never_users             = root
percent_hack_domains    = *
primary_hostname        = admin.cyberenet.net
qualify_domain          = cyberenet.net
receiver_unqualified_hosts = ux1.cyberenet.net:admin.cyberenet.net
#receiver_verify         = true
sender_unqualified_hosts = ux1.cyberenet.net:admin.cyberenet.net
sender_verify           = true
smtp_banner             = "${primary_hostname} -- DO NOT SEND SPAM THROUGH HERE"
smtp_connect_backlog    = 255
trusted_users           = mail
end

local_delivery:
    driver          = appendfile;
    file            = ${home}/.mbox,
    use_lockfile    = false
address_pipe:
    driver          = pipe;
    ignore_status   = true,
    return_output   = true
address_file:
    driver          = appendfile;
    use_lockfile    = false
address_reply:
    driver          = autoreply
smtp:
    driver          = smtp;
end

system_aliases:
    driver          = aliasfile;
    file            = /home/mail/etc/aliases,
    search_type     = lsearch
userforward:
    driver          = forwardfile,
    verify          = false;
    file            = .forward,
    filter          = true
localuser:
    driver          = localuser,
    transport       = local_delivery;
end

lookuphost:
    driver          = lookuphost,
    transport       = smtp;
literal:
    driver          = ipliteral,
    transport       = smtp;
end

* * F,2h,15m; G,16h,2h,1.5; F,4d,8h
end