A client recently reported a problem with 10 user laptops. Users could receive email, but sending email resulting in a 550 5.7.1 error via bounce back. Accounts were configured through Outlook 2010 with SMTP to a corporate Exchange server.
Interestingly enough the laptops were connected to the internet with Verizon 4G modems. They were able to download files, surf the internet, and do pretty much everything else as normal. No settings on the laptop, Outlook 2010, or the Exchange server had changed from one day to the next.
We ruled out the Exchange server by sending from Outlook Web Access as the user. We ruled out the user SMTP account by sending from a stand-alone desktop machine as the user. We then ruled out the laptops themselves by connecting them to a hard wired LAN connection and sending from Outlook 2010 without any problem. The only time we experienced the 550 5.7.1 error was when connected using Verizon 3G or 4G.
Could these recent problems with the Verizon network have something to do with it? We updated the VZAccess Manager software to the absolute latest version (Help / Check for Updates). During that process the firmware of the device was updated as well.
Surprisingly, after the Verizon software and modem updates it was possible to send email again. Hopefully this helps anyone else who is having trouble sending email through SMTP while connected to a Verizon 3G or 4G modem. Upgrade your modem!