Question: I am getting email with partial content or corrupted attachments occasionally, and sometimes multiple copies of such email. Why?

This has something to do with the communication between mail servers (SMTP).
When the sender mail server attempted to deliver the email over to Afteroffice mail server, the email delivery could be failed halfway due to sudden drop on the network quality, or some interruptions in between. At this stage, the email is likely be incomplete and should be ignored by the recipient server, in this case, Afteroffice Mail server.

Afteroffice Mail server engineer will decide to take this further by capturing these incomplete emails, and deliver to our customer account anyway. By doing this, the customer will know that the email has arrived in-time, although corrupted, and serve as an early alert on the email exchange process.
The recipient of the email, in this case, Afteroffice's customer, will be likely getting copies of the same email as the sender mail server will attempt to resend the email again, sometimes over a period of 48 hours.

It is rather annoying if the email carries large attachments and recipient is getting multiple copies of it.
You can always login with the browser, go to Mail and remove the unwanted/incomplete copies of the same email quickly instead of downloading them all.

In some cases, this affects small emails too (without any attachments). When the quality of the network has degraded to a certain level, it affects emails of all sizes.
However, such issue normally would last for a day or two in general, and only affecting specific sender and logistics.

Also, since the email that you sent out is utilizing the same SMTP channel, your outgoing email gets the same “treatment” as well when your connection to the Mail server is less than satisfactory.

This is why you are getting more corrupted email, incomplete email over other service provider when you host your email service with Afteroffice.

What more to be expected:
Afteroffice engineer is working on a way for customer to have this special handling OFF by demand. To follow the usual industrial standard on discarding partial email.
Also, there will be some indications on the email when it was transmitted partially, to avoid confusion over the issue.