Email smtp timeout on mikrotik

RouterOS General
1

Hi All,
Please help me on my outlook issue on mikrotik. Firstly, I would like to explain the situation.

  1. When my internet line from home connect to mikrotik router, my outlook works fine. I could send email with attachment, download new mail with attachment OK.

  2. When my internet line connect from satellite to mikrotik router, my outlook could download new mail with attachment without problem. Sending mail with attachment will cause timeout. Sending email
    without attachment is ok. I have even increase the outgoing mail server timeout, but yet problem still persist.

  3. When I connect my computer to the satellite internet direct, all my outlook works fine. It can send email with attachment without problem. So I could not blame the satellite line for causing the error.

I’ve use the very same unit of mikrotik without any changes, yet the problem happened on satellite internet. There must be some additional settings I need to do to get the smtp to work. The thing that frustrate me is with home internet line, the email works fine.

My email server using pop,smtp with ssl, port 465.

2

Hi All,
Any guide to trouble shoot? Please help.

3

It is unlikely that the MikroTik router is involved in this timing.
Maybe you need to add TCP MSS clamping:

/ip firewall mangle
add action=change-mss chain=forward new-mss=clamp-to-pmtu passthrough=yes \
    protocol=tcp tcp-flags=syn
4

Hi, Thank for the advice. Will try the settings above and let you know. BTW, what are the functions above?

I’m using Mikrotik RB2011UiAS-RM updated with the latest RouterOS.

5

Hi pe1chl,
I’ve tried your settings, the problem is occur. What else should I try? Thanks in advance.

6

No idea. Impossible to debug such problems via forum, especially without config.

7

It needs sniffing to find out what’s going on. Ideally you’d have a second PC, or a second Ethernet interface on the same PC from which you send the e-mail, to which the Mikrotik would stream the sniffed packets. If this is not possible, you can sniff into a file on the Mikrotik itself, but then the attachment must be really small, otherwise the RAMdisk of the 2011 will not be sufficient to hold the complete session.

The sniffing filter has to match only on the IP address of the SMTP server, as matching on port number would prevent packet fragments from being sniffed. And the capture filter on the PC must also not match on port number, for the same reason.

I suppose that when switching from the home connection to the satellite one, you just swap cables and leave the configuration of the Mikrotik unchanged, is that correct?

Would it be possible that the satellite contract assumes that you connect just a single endpoint device, i.e. not a router, to the satellite modem?

8

Well, when the problem is “can send a small mail but it hangs on large mail or attachment” is kind of suggests an MTU issue.
But when “automatic clamp” does not work, it apparently is not an MTU issue that the MikroTik can see (like a PPPoE interface towards internet).
As you say, it would be required to trace it. But difficult to do that for an end user (and also interpret the trace).
Of course a crude extra try could be like this:

/ip firewall mangle
add action=change-mss chain=forward new-mss=1400 passthrough=yes \
    protocol=tcp tcp-flags=syn

(clamp to 1400 instead of clamp-to-pmtu, maybe even try 1280)
However, that is just shooting in the dark.