What are you using as your checkout provider? It doesn't seem like that would cause an issue, but I'll try running a test to confirm that.... so they added numbers in the last name field?
It seems unlikely that it would cause an issue, but let me know your checkout provider and we can create a more accurate test to try to reproduce the problem. That detail may be insignificant, so we may need to look into what else might be the problem. I am not familiar with this issue.
If you have the most recent version of form builder or universal email you could add an email logging action to keep track of failed emails and reasons so you could keep track of any failed emails and the reason they fail.
Either way we will have to be able to reproduce the problem in order to debug a possible solution and confirm it is working, so as many details as possible could expedite that process. If you can reproduce it and we can't then we will have to call and debug it with you directly.