MX record concept

SiLiCoN · 06-17-2005, 03:22 AM

Hi all,

This is not a problem with linux or any specific OS. I am lacking in concept with MX records. Well i would just like to know why an MX record is needed when i mail someone specifying the exact fully qualified domain name.
When i mail someone to a particular domain, lets say "yahoo.com", in that case, the resolver checks the MX record of yahoo.com and sends the mail to the particular users account using yahoo's inbound mail server. I understand the MX record for yahoo.com is required here to let the resolver know about the mail server(s).
now if i mail some one to a particular host (lets say "mr_x@host1.yahoo.com") does the resolver again check for the MX records for yahoo.com? What i want to know is
does the particular computer has to be an "MX record specified" mail server in order to receive mails from other domains even if it is running a mail server (like sendmail or qmail) ?
Please let me know about this a i m trying to figure out the answer for the last couple of hours (maybe i didnt search well in google).

Thanks in advance,
SiLiCoN

stefan_nicolau · 06-17-2005, 05:25 AM

No, if you look at yahoo.com, you will se the following

yahoo.com MX mx1.mail.yahoo.com
mx1.mail.yahoo.com A 4.79.181.15
no need for another MX record.

Also, you can send mail to any host with an A record (or ip)

scowles · 06-17-2005, 05:55 AM

I don't really know if the mail related RFC's specify a delivery order (like MX, then FQDN) for MTA's, but I have noticed that if an MX record is missing, the MTA will try to delvier to the FQDN. i.e. me@host1.mydomain.com.

Note: Just because you can specify a FQDN for the recipient e-mail address, the remote host must also be running an MTA (like postfix) to accept e-mail connections. Furthermore, the remote MTA must be configured to accept e-mail addressed to the FQDN versus the domain name. Most properly configured MTA's will reject e-mail addressed to the FQDN.

Example:

mydomain.com MX record points to mail.mydomain.com

E-mail addressed to me@mydomain.com is accepted, but e-mail addressed to me@mail.mydomain.com would be rejected. Why? Using postfix for example, the "mydestination" parameter does not allow e-mail addressed to the FQDN to be accepted. i.e. from the postfix main.cf file...

Code:

mydomain=mydomain.com
myhostname=mail.$mydomain
mydestination = localhost.$mydomain, localhost, $mydomain

but... if I configured the "mydestination" parameter to include the FQDN (myhostname), then e-mail addressed to the FQDN would also be accepted. i.e.

Code:

mydomain=mydomain.com
myhostname=mail.$mydomain
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain

SiLiCoN · 06-17-2005, 06:57 AM

thanks for the replies
i ll tell you exactly what happened....
i have access to a subdomain for a large site (lets say xxx.com and my subdomain as me.xxx.com)
now i had set up qmail on "me.xxx.com"
it was able to send mails to gmail and gmail recieved it with username@me.xxx.com
when i replied to the mail (from gmail), first of all the messages bounced back saying "mail cannot be delivered to me.xxx.com -SMTP relay access denied" - i know that problem has got to do nothing with MX records
now i had set the MX record of xxx.com to me.xxx.com ,
now when i send mails from gmail to me.xxx.com, the error message did not appear and no mail delivery failure message was recieved in the gmails inbox (strange: the SMTP relay access denied problem just disappeared)
but at the server (me.xxx.com) the mails were not there - i didnt find any mails in that server and the mails seem to just disappear

any clues?