Why do some form autoresponses go to "junk mail"

We've had this happen a couple of times: a user completes one of our forms and the autoresponse that is our notification goes into our  junk mail folder. This happens only rarely, but it does occur. It has happened with several of our forms, but there seems to be no pattern. 

Any ideas? (It seems that if our mail server were blocking autoresponses from JotForm, ALL would be block, not just the occasional one.)

Thank you for contacting us.

If you use a custom sender, it is recommended to switch to one of our standard senders or your own SMTP sender instead. When you use a custom sender the emails are being sent from our servers on behalf of your domain which might be considered as not safe by some email providers.

If you are not getting emails to some address, please check it against our bounce list first.

How to Remove Your Email Address from Bounce List

Please let us know if you need any further assistance on this.