Large attachments can not be sent

Hello Lyndon.

This usually is an indication there is an MTU (Maximum Transmission Unit) setting problem.

Are you connected directly to the modem via Ethernet of thru a router connected to the modem?

The default MTU setting with OS X is 1500 which is the highest available.

You may need to check with your ISP regarding any changes or for their recommended MTU setting.

In the interim, try the following.

Go to System Preferences > Network.

At Show:, select Built-in Ethernet.

Under the Ethernet tab at Configure, select the pop-up menu and change Automatically to or select Manually (Advanced).

At Maximum Package Size (MTU), select Custom.

Enter 1492 in the space provided and select Apply Now when finished to save the changed settings.

Test if this resolves the problem.

Regarding your Airport Extreme, did you check with your ISP regarding the required settings?

I'm sort of having the same problem. this is my story.

I am having trouble sending an mp3 file to the accounts of folks I know. I can send jpeg files to them, and ordinary eMail but when I try to send an mp3 I get the following warning:

"Cannot send message using the server
smtp.west.cox.net

Use the pop-up menu below to try a different outgoing mail server. All messages will use this server until you quit Mail or change your network settings."

I have sent mp3 files this way before but now I cannot. The mp3 file is only 7 Megs and I'm using COX Broadband. Anyone know why?
Thanks

I have been having the same sort of problem since installing Tiger in early July. My attachments are all >20K, but much less than my 10M server limit. I have Direcway 6000, G4 DP.

Note that I have 3 macs on the network, and the problem only occurs on the one system upgraded to Tiger. Something happened in the upgrade to Tiger mail that causes this problem.

Currently MTU is set to 1492 with no change in results. In desperation, I turned on some debugging flags in PostFix and checked mail.log. I have attached the part that looks like it descibes the error:

Sep 12 07:25:19 William-Kirkhams-Computer postfix/smtpd[292]: > localhost[127.0.0.1]: 250 Ok
Sep 12 07:25:19 William-Kirkhams-Computer postfix/smtpd[292]: watchdog_pat: 0x308a68
Sep 12 07:25:19 William-Kirkhams-Computer postfix/smtpd[292]: < localhost[127.0.0.1]: DATA
Sep 12 07:25:19 William-Kirkhams-Computer postfix/smtpd[292]: > localhost[127.0.0.1]: 354 End data with <CR><LF>.<CR><LF>
Sep 12 07:25:19 William-Kirkhams-Computer postfix/cleanup[295]: 2F392E65292: message-id=<0759743A-96CC-4D8D-BC0A-3A0C8866DADA@direcway.com>
Sep 12 07:26:34 William-Kirkhams-Computer postfix/smtpd[292]: smtp_get: EOF
Sep 12 07:26:34 William-Kirkhams-Computer postfix/smtpd[292]: lost connection after DATA from localhost[127.0.0.1]
Sep 12 07:26:34 William-Kirkhams-Computer postfix/smtpd[292]: disconnect from localhost[127.0.0.1]
Sep 12 07:26:34 William-Kirkhams-Computer postfix/smtpd[292]: master_notify: status 1
Sep 12 07:26:34 William-Kirkhams-Computer postfix/smtpd[292]: connection closed

Any other ideas?

--Bill

Does Cox have an overall message size limit for incoming and sent messages that may have been changed or reduced recently?

A 7MB file attachment will be over 10MB after the message encoding process and if Cox has a 10MB overall message size limit, this is why such a message cannot be sent using Cox's SMTP server.

Check What is the maximum size of email attachments? regarding the message encoding process.

Hello William.

Have you contacted your ISP regarding their recommended MTU setting?

What you are experiencing is usually caused by an incorrect MTU setting and changing the OS X default setting of 1500 to 1492 may not be enough if the MTU setting is the cause.

Thanks Allan. I have sent them an email to find out. Normally this would be to tough a question for their standard support staff in India.

I checked an iMac running 10.3.9 on this network. Ethernet is set to "automatic" but when I set it to "Manual" it shows an MTU of 1500. It has no problems sending large attachments.

BTW, I was able to send an email with a 9 KB size. Much smaller than my typical attachments. Also, this happens on only one of three macs on this network, and the only one with a problem is the G4 DP which was upgraded to 10.4.2. A new Powerbook G4 came with Tiger installed and has not had this problem.

Allan,
I calld COX today and they told me exactly what you told me. So that was the problem. My 7 Meg file was just slightly above 7 Megs. I guess things are never what they appear to be! I would have thought 7 Megs was 7 Megs!

thanks for your help!

Allan--

Direcway has confirmed that the MTU setting for a DW 6000 system is 1500. I also tried a value in the 500s that I heard worked best for dialup lines without any change in performance.

The problem is odd in that it seems to happen during the upgrade process. Initially I thought it was a permissions issue, but "Postfix check" seems to think things are okay, and I have used Disk Utility to repair permissions also. If you can think of any other possible solutions or diagnostics, let me know.

Thanks!

--William