@crun, for me this did actually help, on Xperia 10 iii.
It says “sending…” but has not actually sent the message (unlike the OP).
The following workaround seems to work:
- Deleting the message
- Switch from Mobile Network >> Prefer 4G to Prefer 3G. Wait a few seconds for the change to take effect.
- Re-send the message
- After it is sent, switch back to Prefer 4G
Not sure if it will work for anyone else.
Thanks.