[JimDabell]

This is very unlikely because it seems to be something that happens in the official client rather than server-side. Spammers are unlikely to use official clients and blocking them there doesn't make any sense.

Having said that, if they were intentionally trying to block this then using their anti-spam mechanisms would be an obvious way of doing so, so I think it's definitely a possibility that this is some accidental side-effect of something else rather than a simple intentional block.