Open the door of your front-loading washer. There is a rubber sealing gasket that seals around the front door. Grab a bucket of bleach water and a rag and wipe out around the rubber gasket, paying close attention to the middle of the gasket where water tends to settle. Leave the door open and allow the gasket to air dry.
Many front-loading washers come with a cleaning cycle built in. Fill the unit's bleach tray with bleach and run through a cleaning cycle. If your front loader does not come with a cleaning cycle, fill your washing machine's bleach tray with bleach and run a normal, empty cycle. This will sanitize and freshen the unit. After the cycle has been ran, prop the door open and allow the washing machine to air dry.
Many front-loading washing machine manufacturers recommend leaving the front door open after running a load of landry to allow the front gasket to air dry.