Event Receiver not deploying newest version

I have a custom event receiver written for a document library that triggers on ItemAdded. The intent of this library is that users email documents into it rather than use the UI to upload, and when I initially started to test this event receiver, I used both the uploading function and the email function by dropping .eml files into the mailroot\Drop folder on the SharePoint server to simulate an email. Both of these worked fine for a while, but now I'm noticing that the email function seems to be triggering an older version of the event receiver while if I use the upload function, it works fine. I don't have multiple versions running simultaneously so it's like it is caching a previous version. What can I do to update it?