I’m not sure how many messages I was forwarding, but it was somewhere around 100 or more. I was simply forwarding them to a mailbox with unlimited storage - the first one being near capacity. After stopping to get a cup of coffee I returned to find a pop-up saying my session had expired and I needed to log back in… which I couldn’t. And I couldn’t on any of my domains with the same first level domain from that registrar. It wasn’t until I checked Whois that I found out I (the registrant) was “disabled for abuse.” 
Eventually I was “restored” and thereafter limited myself to no more than 10 forwards an hour. 
They’re just trying to stop spam. You could probably contact support for your domain registrar and/or webmail provider and ask them what the limit is. They may tell you… or not. 