Some ISP’s require you to use their SMTP server to send with. At these wireless hotspots, that is probably the case. The SMTP you are using isn’t theirs, therefore they won’t let the information pass. I know Cox Communications will only allow you to send email on their network using their SMTP servers (This is for home and small business). Is it the same from any and all wireless hotspots? If so, it could be your ISP not allowing relaying through them from another IP.
In your SMTP server configuration look for something like “My server requires authentication”. Enter your ISP login credentials. Some ISP’s use a non standard port so check with your provider.