When sending to [email protected] via PHP mail() function, I receive mails. When sending emails from external machines, I receive the following (e.g., sending from [email protected]. [mail.ru is Russian gmail]):

This is the mail system at host fallback2.mail.ru.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<[email protected]>: lost connection with mail.mydomain.com[xxx.xxx.xxx.xxx]
    while receiving the initial server greeting

Reporting-MTA: dns; fallback2.mail.ru
X-mPOP-Fallback_MX-Queue-ID: D8C19F2411F1
X-mPOP-Fallback_MX-Sender: rfc822; [email protected]
Arrival-Date: Tue, 29 Oct 2013 10:09:21 +0400 (MSK)

Final-Recipient: rfc822; [email protected]
Original-Recipient: rfc822;[email protected]
Action: failed
Status: 4.4.2
Diagnostic-Code: X-mPOP-Fallback_MX; lost connection with
    mail.tld.com[xxx.xxx.xxx.xxx] while receiving the initial server
    greeting

Here is my postfix main.cf:

command_directory = /usr/sbin
daemon_directory = /usr/libexec/postfix
data_directory = /var/lib/postfix
myhostname = mail.mydomain.com
mydomain = mydomain.com
myorigin = mydomain.com
inet_interfaces = all
inet_protocols = all
unknown_local_recipient_reject_code = 550
in_flow_delay = 1s
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mail_name = mydomain.com daemon
debug_peer_level = 2
debugger_command =
     PATH=/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin
     ddd $daemon_directory/$process_name $process_id & sleep 5
sendmail_path = /usr/sbin/sendmail.postfix
newaliases_path = /usr/bin/newaliases.postfix
mailq_path = /usr/bin/mailq.postfix
setgid_group = postdrop
html_directory = no
manpage_directory = /usr/share/man
sample_directory = /usr/share/doc/postfix-2.6.6/samples
readme_directory = /usr/share/doc/postfix-2.6.6/README_FILES

bounce_queue_lifetime = 4h
maximal_queue_lifetime = 4h
delay_warning_time = 1h
strict_rfc821_envelopes = yes
show_user_unknown_table_name = no
allow_percent_hack = no
swap_bangpath = no

smtpd_delay_reject = yes
smtpd_error_sleep_time = 20
smtpd_soft_error_limit = 1
smtpd_hard_error_limit = 3
smtpd_junk_command_limit = 2
mydestination = mydomain.com, localhost.localdomain, localhost
smtpd_client_restrictions = permit_inet_interfaces
smtpd_recipient_limit = 100

virtual_alias_domains = mydomain.com
virtual_alias_maps = hash:/etc/postfix/virtual

smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination

Why emails from external server are not being delivered? Thank you!

Update

In a log, the following lines appear a lot of times

Oct 30 10:48:29 mydomain postfix/smtpd[16216]: connect from fallback5.mail.ru[94.100.176.59]
Oct 30 10:48:29 mydomain postfix/smtpd[16216]: warning: SASL: Connect to private/auth failed: Connection refused
Oct 30 10:48:29 mydomain postfix/smtpd[16216]: fatal: no SASL authentication mechanisms

It appears I have to configure SASL? I would understand if I would like to send emails from postfix, but why do I need it to receive emails?