Intermittent 5.7.1 email bounce to Exchange 2007

My knowledge of Exchange isn't particularly great, so excuse me if some of the terminology I use isn't quite right. I'm primarily a web developer who's now responsible for a small business's network.

We have a server running SBS 2008 and Exchange 2007. Generally, everything works well, emails are able to be sent to both internal and external domains without issue. We've only got ~20 users, Exchange is sitting on a single server.

I use SendGrid to send emails generated by our externally hosted website to users in the office. Primarily, order notifications are sent to orders@somedomain.com.
Without any pattern and less than once per week on average, an email to orders@somedomain.com will bounce back, and the logs on SendGrid detail the following error:

550 5.7.1 Unable to relay for orders@somedomain.com

Either side of that failed delivery attempt, I'm able to send and receive emails to/from orders@somedomain.com.

Having done some research, incorrect reverse DNS seems like it could be a cause of intermittent bounces like this. Having used nslookup, I have found that the reverse DNS doesn't map like it should, e.g.

Office IP: 135.325.351.123 (made up IP, for example only)
Domain: office.somedomain.com (made up, for example only)
Reverse DNS: somedomain.gotadsl.co.uk (half made up)

Could this be a cause? I'm sure that the IP address and the domain should map to each other.

Also, it has been suggested to me that as the Exchange server is on a network with an ADSL connection, that could be a potential cause as the connection "goes up and down all day long". I don't have an opinion on this, as I don't have enough knowledge of Exchange/ADSL to form a reliable opinion.

Can anyone offer any insight as to whether one or both are actually potential causes, or if there is another possible cause?

Answer

Both comments led to me finding the solution. Yes the ADSL will cause me issues, and I did have a faulty MX record set up for a backup mail server, which has since had it's IP changed. The answer for my scenario is to use a backup mail server to allow emails to still be delivered to the domains I manage even when my internal server cannot be reached (due to the ADSL connection). This backup mail server will be external, and will be a paid-for service (e.g. MxSave with a SLA).