Operating system name and version: openwrt
Version of smsd: 3.1.21
Smsd installed from: sources
Name and model of a modem / phone: n/a
Interface: n/a

I need smstools 3 retrying sending SMS for 10 minutes whatever error happens. Tried errorsleeptime=10 and send_retries=12 and check_sim_retries=-1 with SIM card removed, and SMS goes into FAILED state within 30 seconds.

How to achieve the goal? Thank you!

There is no built-in function for this, so the function has to be done with an eventhandler that you have to create yourself. There are no ready-made scripts for this, so I will only describe the main features of the operation:

Use smstools factory settings and add general settings:
failed = /var/spool/sms/failed
eventhandler = /path/to/eventhandler

When the message is processed, smsd calls the eventhandler. When the first parameter is "FAILED", read the header "RetryStart:" from the file (the second parameter). If the header is found, count the time and exit if necessary. If the header is not found, add it with the timestamp to the file. Move the file back to the outgoing directory and smsd will try sending again.

Thank you very much!

Apologies, I still have an issue, but in different place.
I implemented as you advised, and it works well.
But smstools says the following:

Fatal error: sending failed 3 times. Blocking 3600 sec.

and stops sending. I did not wait an hour, but suspect next time it will try sending is in 1 hour. But the task is to make smstools trying sending SMS as hard as possible until connectivity restores (e.g. in one or two minutes) or 10 minutes elapse. This 1 hour sleep breaks the whole algorithm...

blocktime=10

into the device section seems to be a solution!