Outgoing mail with attachments - load balancing Draytek

We've picked up a new customer who have frustrations sending email with attachments.  

They have a number of Macs using Mac Mail.  They have an existing Draytek Vigor 2960 router which load balances between a laggy satellite connection for nearly all traffic, other than IMAP and SMTP which is routed through a DSL connection via a Policy Route.  Tracert to their mailserver proves this is setup correctly and simple messages flow out of the building successfully.

The problem comes when sending attachments of say 500KB or above.  These time-out and get stuck in their outgoing mail nearly all the time.

I suspect that although the route policy is set up correctly (confirmed by Draytek as well as tracert) some other associated traffic is being routed via the satellite.  This was partially confirmed when the satellite connection failed and all traffic failed-over to the DSL connection.  At that point, mail with attachments were sent successfully.  It all started when the satellite connection became unreliable.

Any ideas what other traffic I should be re-routing via the DSL connection? I thought maybe DNS for MX lookups but surely that would affect all SMTP traffic, not just messages with attachments?  We've also tested it with using port 25 & 465 - there's no improvement.

Thoughts before I head back over?  Thanks, Ian.
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Usually in these cases, I just assume the route is correctly set up indeed, and use Wireshark on the computer to follow the whole SMTP conversation. SMTP is quite simple and really travels over one port (unlike FTP/passive), so I don't think it's a routing issue.
DNS is not an issue if you're not the mailserver, and even then, it fails at the very beginning (not midway).
blueskycomputingAuthor Commented:
Sorry for not getting back sooner.  It turned out to be a faulty router.  Replacing the hardware fixed it immediately

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blueskycomputingAuthor Commented:
Faulty hardware was the issue
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