Trying to send a blueyonder email from my iphone on 4G, I get 'no password provided' even though it is there correctly in the email settings on the phone, and it will work if I am on wifi. This has happened before but I ignored it usually because it works ok if I am on wifi. I have tried deleting the email account (blueyonder) on the iphone and reinstalling, even restarting the phone. I also tried retyping the password in the outgoing settings, also with restarting. Still does not work!
Most phones don't give a useful error message - they just report they got stuck, but don't feed back the VMxxx error code - which is essential to give you accurate diagnosis of what is causing the problem and how to solve it.
Can you accesswebmailin a browser (on your phone or any other device)? Can you send and receive through webmail? Webmail uses different servers so it is useful to know if that is working normally.
Another thing we should rule out is whether the IP address when your phone is using 4G connection is on a blocklist. VM uses blocklists to reduce improper traffic.
To find your phone's IP address, go to Settings > About device > Status. Your phone or tablet's IP address will be displayed with other information, such as the IMEI or Wi-Fi MAC addresses: Mobile operators and ISPs also provide a so-called public IP address.
Enter that IP address in the spamhaus query page https://www.spamhaus.org/lookup/. Tell us which lists it appears in (but don't tell us the IP address). We can give more advice once we know the status of the IP address.
IIRC, phone IP addresses are shared with other users, so you can't control what others may be doing and it isn't easy to tell if your device is causing the listing or someone else's device.