Using secure SMTP on port 537

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Level 1

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Using secure SMTP on port 537

Hello,

I work in the NOC for a cable ISP, where we allow our customers to use secure SMTP on port 537, if they wish to send email when away from home, on another network. This has worked fine in the past for both Mac and Windows, and continues to work for Windows. However, we have recently received reports from our Mac customers of not being able to use this feature.

Today, we tested this extensively, even going so far as configuring a modem without any blocked ports just to make sure we weren't overlooking something in the config. However, at this point we can consistently send on port 537 from Outlook Express or Mozilla Thunderbird in Windows XP, and get nothing but errors when doing the same from Mac Mail, MS Entourage or Mozilla Thunderbird in Mac OSX 10.5. Thunderbird gives the most detailed error messages, specifically saying that the server does not support authentication, even though the exact same settings work in Thunderbird for Windows.

I've seen this work on the Mac previously, not only on my own Mac laptop, but on my iPod Touch. Last year I configured the Mail program on my Touch to use the same mail server with SMTP authentication on port 537, so that I would be able to send email no matter what wifi network I was using. It worked fine when I set it up, and while I have not actually used tried sending mail from my iPod for months, when I tried it today as part of this testing, I encountered the same connection errors as from Mac Mail on OSX.

Whatever has changed, it changed within the past six months. We created a custom configuration file for one of our modems, specifically to eliminate any factors that could have changed on our network. Now, we're stumped. I can sit on the mail server and watch the logs, and the SMTP connections from the Windows test machines go right through, while the attempts from the Mac's don't even log as a connection in the logs, so it's not even reaching the servers. There are no problems receiving mail, or sending with regular SMTP on port 25.

When searching the support forums here, the closest I could find to relevant posts were an issue that appeared to be specific to postfix, and a post from almost two years ago that referenced Bug ID# 4818688, but that issue was from before we had this SMTP method working on Mac.

Any thoughts?

Multiple, Mac OS X (10.5.6)

Posted on Apr 24, 2009 4:48 PM

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Answer 1

j.v.

Level 5

4,155 points

Apr 24, 2009 5:59 PM in response to Cable NOC

I don't do this for my profession, but I do have my own small personal mailserver running at home. My mail clients send smtp traffic to a 5-digit-numbered port on smtp server localhost, which is, in turn, tunneled through ssh to my home network. That requires, on Apple Mail client, Preferences > Accounts > smtp server > edit server list > advanced and choose "Use custom port." Are your clients configuring Mail.app similarly?

Since my clients' stuff is tunneled through ssh, I don't require the use of SSL with Mail. So, I also wonder whether there might be some sort of issue with an SSL cert having been created while a different port number was being used than the port number you are using now. I've seen a similar sort of thing happen with ssh keys if you change the ssh server's port number over which you connect.

That's my two cents worth. If nothing else, I hope it spurs some additional ideas for you to pursue. Good luck! And please post back your solution, once found.

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Answer 2

Cable NOC Author

Level 1

0 points

Apr 27, 2009 7:58 PM in response to j.v.

I wish it was just a matter of the right settings in the mail client. However, I was working with another NOC engineer and one of our IP admins for hours, testing every possible setting in Mac Mail, Entourage and Thunderbird. Using SSL, Password authentication, SPA off, port 537, it works on Outlook Express and Thunderbird for Windows. Using the exact same settings, no mail client works on the Mac. That's why we reached the point of creating a custom configuration file on a modem, packet sniffing at each point in the connection and monitoring the SMTP logs... to make sure there wasn't something on our network that was interfering with Mac's that would not affect Windows.

It's not just a matter of getting the mail client settings correct, otherwise the exact same settings that work for OE would work in Entourage, and the exact same settings that work for Thunderbird for Windows would work in Thunderbird for Mac.

Thunderbird for Mac was the most helpful in trying to track this down, because it said that the server did not accept the authentication request. However, the SMTP server shows that no attempt for authentication from a Mac even reached the server, and as far as we can tell, the attempt at password authentication on port 537 never even left the Mac and reached the router or modem, for any mail client used.

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