Websites (or, more precisely, sendmail) for domains that have their MX record set differently than their A record have a problem with sending mail to addresses in their domain. I found this potential solution on the web:
Option 5: THIS IS WHAT WORKED FOR ME add a LOCAL_RULE_0 to the sendmail mc file
LOCAL_RULESETS LOCAL_RULE_0 R$* < @domain.com. > $* <tab>$#esmtp $@ mail.domain.com $: $1<@domain.com.>$2
this needs to go at the end of the file and <tab> needs to be an actual tab character
So, the question is, how do we find and edit the sendmail mc file on an Arvixe host?