Surely at on point you've used an automated system to send messages via Google Mail (GMail). Eg, using ssmtp with Google Mail. We do it too, but have noticed that many times Google Mail will report a successful receipt but never actually delivers the message.
I wonder how many communications have been lost due to this?
Note: this happens when using their Gmail or Google Apps systems - Standard or "Enterprise".
Symptom Mail Logs
Here is a snip from a verbose mail log, which shows Google reporting successful receipt of a message. Names have been changed to protect the innocent.
~ # echo -en "Subject: Test Demo 2326\n\nTest Demo Machine" | sendmail rcpt@edoceo.com sSMTP[15508]: Set HostName="demo.edoceo.com" sSMTP[15508]: Set Root="host@edoceo.com" sSMTP[15508]: Set MailHub="smtp.gmail.com" sSMTP[15508]: Set RemotePort="587" sSMTP[15508]: Set FromLineOverride="True" sSMTP[15508]: Set AuthUser="sender@edoceo.com" sSMTP[15508]: Set AuthPass="complicated-password-here" sSMTP[15508]: Set UseSTARTTLS="True" sSMTP[15508]: Creating SSL connection to host sSMTP[15508]: 220 mx.google.com ESMTP hh2sm2028847obb.1 sSMTP[15508]: EHLO demo.edoceo.com sSMTP[15508]: 250 ENHANCEDSTATUSCODES sSMTP[15508]: STARTTLS sSMTP[15508]: 220 2.0.0 Ready to start TLS sSMTP[15508]: SSL connection using RSA_ARCFOUR_SHA1 sSMTP[15508]: EHLO demo.edoceo.com sSMTP[15508]: 250 ENHANCEDSTATUSCODES sSMTP[15508]: AUTH LOGIN sSMTP[15508]: 334 XXXXXXXXXXXXXX sSMTP[15508]: XXXXXXXXXXXXXXdmlzdGNlbnRyYWwubmV0 sSMTP[15508]: 334 XXXXXXXXXXX sSMTP[15508]: XXXXXXXXXXXXXXNpZ2FyLXBpenph sSMTP[15508]: 235 2.7.0 Accepted sSMTP[15508]: MAIL FROM:<sender@edoceo.com> sSMTP[15508]: 250 2.1.0 OK hh2sm2028847obb.1 sSMTP[15508]: RCPT TO:<rcpt@edoceo.com> sSMTP[15508]: 250 2.1.5 OK hh2sm2028847obb.1 sSMTP[15508]: DATA sSMTP[15508]: 354 Go ahead hh2sm2028847obb.1 sSMTP[15508]: Received: by demo.edoceo.com (sSMTP sendmail emulation); Wed, 11 Apr 2012 06:37:14 +0000 sSMTP[15508]: From: "root" <root@demo.edoceo.com> sSMTP[15508]: Date: Wed, 11 Apr 2012 06:37:14 +0000 sSMTP[15508]: Subject: Test Demo 2326 sSMTP[15508]: sSMTP[15508]: Test Demo Machine sSMTP[15508]: . sSMTP[15508]: 250 2.0.0 OK 1334126238 hh2sm2028847obb.1 sSMTP[15508]: QUIT sSMTP[15508]: 221 2.0.0 closing connection hh2sm2028847obb.1 sSMTP[15508]: Sent mail for root@demo.edoceo.com (221 2.0.0 closing connection hh2sm2028847obb.1) uid=0 username=root outbytes=428
Notice the line right before QUIT, where google reports 250 2.0.0 OK 1334126238 hh2sm2028847obb.1.
Wait as long as you like, the message never arrives. It doesn't go to SPAM, it doesn't get accidentally labeled as something else. It just disappears.
I've run this same test agains multiple target accounts in Google Mail systems, across multiple domains. I've run this test from various domains, from various servers and server-configurations.
However, I cannot predict when/how this will happen. So far it's happend with mulitiple SMTP softwares including ssmtp (as above), postfix, PEAR::Net_SMTP, radix_mail_smtp and also seen this occur from the Gmail application on an Android device. Haven't had a report of this from iPhone (yet) - but we just started tracking.
Also note, the issue is not consistent. For example the test message from above failed, but after I went to have a pint across the street and came back message would be delivered. Previous messages were still lost.