Thank you for reaching out to us in our community, in order to try to get to the bottom of this there is a few things from them to check.
1. Is it always to the same addresses
2. Is it always within the same hour - if they wait until the next hour can they then send to those addresses
3. If it is to all addresses then if they wait until the next hour are they then able to send to those addresses
1 and 2 together indicate that it is the recipient's ISP or some other point at their end (SPAM firewall?) that has the restriction.
3 indicates that it is probably either their ISP or other point at their end end (SPAN Firewall?) with the restriction.
The bounces occur anytime mail is sent to a recipient at ntlworld.com, virginmedia.com, virgin.net, blueyonder.co.uk or any other Virgin Media domain. This would seem to indicate Virgin Media is the one throttling the mail. Here is the format of the SMTP responses:
421 4.1.1 logid=SMTPRC 421 MXIN503 Hourly ratelimit for your IP exceeded ;id=AWVhjLA2wxWCGAWVijB69A;sid=AWVhjLA2wxWCG;mta=mx11.tb;d=20200307;t=111646[CET];ipsrc=xx.xx.xx.xx