It is pretty simple really the domain you are sending from jerseycatering.com does not meet the requirement set by Google for email senders. So they are bouncing the mail.

That is essentially what it says in this link in your question. https://support.google.com/a/answer/81126?visit_id=638417489267895194-3535439807&rd=1#authentication

When I checked the IP address that Google were complaining about I find it is altprdrgo05.altice.prod.msg.synchronoss.net [65.20.63.221]

I did try checking for some of those items google require. I see none of them https://mxtoolbox.com/SuperTool.aspx

So I guess whomever is responsible for the configuration of the domain at your work will need to have a discussion with Hover about what changes need to be made. Lots I think by the looks of it.

The emails only get returned when I use Thunderbird. If I log into my Hover email site I can send emails to gmail accounts without them being returned. So it's something with Thunderbird

So it's something with Thunderbird

No, it is not.

Google say jerseycatering.com has no assorted server side validations setup. I manually checked that and it was not there for the server. Have you tried checking using the links I provided, or are you just shooting from the hip. Failed SPF check on jerseycatering.com https://www.spf-record.com/spf-lookup/jerseycatering.com?opt_out=on Failed DKIM check for jerseycatering.com https://mxtoolbox.com/SuperTool.aspx?action=dkim%3ajerseycatering.com%3aemail&run=toolpage

failed DMARC check for jerseycatering https://mxtoolbox.com/problem/dmarc/DMARC-Record-Published?page=health_dmarc&hidetoc=0&hidepitch=0&showlogin=1&action=dmarc:jerseycatering.com&domain=jerseycatering.com

Failed SPF check on Hover.com https://www.spf-record.com/spf-lookup/hover.com

I have no idea what server Hover use to send mail, but clearly it is not the one they tell you to use to send mail from mail clients if you can send from their web portal but not from a mail client. Regardless of what you think. The problem is at Hover. They (google) are not complaining about your mail client, or your home IP address. They are complaining about a specific sender addresss of 65.20.63.221 which as I said earlier resolves to altprdrgo05.altice.prod.msg.synchronoss.net

There is an invalid SPF record for synchronoss.net which probably does not help things, https://www.spf-record.com/spf-lookup/synchronoss.net All this is a direct result of Googles announced tightening of their deliverability standards. Hover do not meet them (I assume this synchonoss are providing services to Hover, it is the only way I can get them into the email equation) and until you ask Hover for help I doubt you will make any progress at all. You are certainly not going to make any here.

Thunderbird has no involvement in DKIM, DMARC SPF it is all managed at a server level using DNS