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 email@example.com. Without any pattern and less than once per week on average, an email to firstname.lastname@example.org will bounce back, and the logs on SendGrid detail the following error:
550 5.7.1 Unable to relay for email@example.com
Either side of that failed delivery attempt, I'm able to send and receive emails to/from firstname.lastname@example.org.
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?