We just released a new patch which fixes an important federation problem. Due to a mistake in the code, activities were only sent out once, even if delivery failed (e.g. because the target instance is down, or a temporary connection issue). Instance admins should definitely upgrade.
This problem is fixed, so that failed deliveries are retried up to 9 times, with exponentionally increasing intervals. The only limitation now is that it will not keep retrying after Lemmy is restarted, but that case should be less common.
Yeah, according to logs the POST requests are going through fine.