Is something failing at the incoming mail server?

[#1]

I've emails set up to be forwarded to an address @ my plusnet account.

I notice an email sent to me today hasn't arrived and checking the forwarding email, on the 15th, yesterday, and today, at totally different times, it's reported Mail Forwarding Failure for different emails, status 421. This sort of thing.

It LOOKS like they are trying and something is wrong somewhere. As I say, 3 reports on last 3 days (not including the one that didn't arrive today and I'm guessing might be similar). Other emails are getting through, it's totally intermittent? This has never happened before that I'm aware of and I've changed no settings.

Thought I'd mention it in case others aren't getting emails.

Technical details of the failure: 
Host not reachable.

Original-Recipient: rfc822; DELETED
Final-Recipient: rfc822; DELETED BY ME
Status: 421
Action: failed
Last-Attempt-Date: 17 Dec 2024 00:37:44 GMT
Diagnostic-Code: Host not reachable.
Remote-MTA: dns; mx.avasin.plus.net 

[#2]

Where is this mail being forwarded from?

“SMTP Error 421 is typically a temporary issue related to server overload, network problems, or configuration errors. It's not a direct indication of being blacklisted. However, if you frequently encounter this error along with other delivery issues, it may be worthwhile to check your IP's reputation.”

See https://blog.warmy.io/blog/how-to-fix-smtp-email-error-421-solved/#:~:text=SMTP%20Error%20421%20is%2....

[#3]

These are getting forwarded from Zoho who are a fairly big email provider?

It's random if it does this, seems to be no pattern.

The email I was expecting eventually arrived at plusnet about 3 hours late, another later didn't with the same error.

As I say I can't see this happening until about 3 days ago. What stands out is it's a straight up "host unreachable", not a complaint about SPF or DKIM or viruses or anything like that.