It's pretty terrible at handling any kind of networking failure (or whatever else causes it to have trouble getting a message delivered). Sometimes the UI will make something appear to have been sent, and quite some time later it gets marked as unsent. There's no explicit option to quit trying, or retry. Sometimes that message eventually disappears, and sometimes the recipient will then receive numerous identical copies of the message without me taking any action. This seems to be typical among users at other companies I talk to as well. This also seems to happen a lot more than I would expect, given that Internet connections are pretty reliable these days.