There are a multitude of reasons it could happen.
The most likely candidate is a spam filter server side. I will use Gmail as an example, but most are similar.
If you set up with Gmail, then send 3 or more emails in succession, it will usually reproduce your outcome if done in less than 5 minutes. This can be done from the webclient as well if you send like the machine does, which is to say:
Every email sent from a printer is essentially identical with exception to the attachment size.
The subject line, message body, attachment name, footer are all identical every single time. So if you send the identical message over and over in a short period, the Mail server will typically look into the sender and slow, or block communication from it. The printer does not understand the messages it is getting back from the mail server, and it is very hard to program it to do so as each one is slightly different.
So you reboot the printer and the jobs go through, but it is mostly the delay that fixed it, and the fact that it retries the same message upon boot. Typically you can just delete the processing job, wait 5 minutes and send the same scan again as well.
Many have luck by just changing the File name on every 3rd scan when doing large batches of jobs which is done on the Email > Email Options > File Name screen.
Of course this may not be your current scenario, but it is what is seen most often by myself as well as friends who deal with other manufacturers.