Are you trying to send a DM? If so, it will send messages to the counterpart’s DM relay list. If you can’t connect to the counterpart's DM relay list, the result will be a sending failure.
Discussion
I have 2 phones with separate Nsec. Both connected the same way to the same servers. I should be able to send a DM from one phone to another via my private server. That’s what I think anyway. But it keeps failing. I have tried multiple settings, with Orbot on and off. If I use the supplied DM servers then the message goes through. Trying to use my private server fails.