That's interesting. I often get email meant for someone else on my gmail account. They don't seem to be spam, but always from business and normal website accounts. The address I see on the messages is definitely my address. The best I can see is the other person forgets to add whatever number goes on the end when they give the address to someone. The other explanation is some email server is dropping characters from the address. That seems very unlikely.

In your case, you seem confident that the correct address was used, but the number was dropped off the end and then sent to a matching address. Obviously, the place to start is to check the sent message since you have direct access. Just make sure the correct address was used for those messages. In other words, you are trying to isolate the issue to the sending computer, or your wife's email provider, or gmail's infrastructure.

If the address is correct on the local computer, you should be able to send a test message and see where it goes. You might have some help if the other person is willing to play ball in the diagnostics.