No, not really, but I'd guess it's a problem with the ISP's mail server. Are you able to set up your SME server to act as your mail server directly (that is, not relaying through your ISP)? Failing that, I'd suggest checking the phpbb.com forums, as there might be a way to get more detail about what's happening.
Just noticed something, though--if you've set the SME server (using the server manager panel) to relay through your ISP's server, just set phpBB to use 'localhost' with no username or password as the SMTP server.