In our experience, this is most likely to occur on mobile devices; the smaller screen sizes can make it hard to tell exactly which email address the autocomplete suggestion has picked as the recipient of your message. Particularly when you’re on the go, you may not notice that autocomplete has selected the wrong address and is sending a message intended for your OmniFocus inbox to some other party.
We’ve found that it can be helpful to create a “Mail Drop” contact and add your custom address to that contact. You can find your custom Mail Drop address by logging into your account on the server.
We’d additionally suggest omitting “OmniFocus” or “Omni Sync” from the name of that contact; it’s not uncommon for autocomplete to match to one of our email addresses you’ve previously corresponded with. (We don’t mind receiving those emails, but we figure most of our customers would prefer to keep information to themselves whenever possible.)