Next time it happens open a ticket at the helpdesk as soon as it happens and provide us with the problematic email addresses and we can check the firewalls here.

I was suggesting perhaps there is a block on this end because if your mail program is set to check for emails too frequently, the server may temporarily blacklist your IP for a short while.

Typically if your mail program is set to check every minute it is too frequent, especially if you have multiple machines on the same public IP set this way.

If the above applies to you, you might try a setting like every 5 minutes or so.