Basically, your target address uses Mimecast protection which is blocking the sending address or server.
Mimecast website says -
“A 5xx error code is returned if the server has encountered a permanent error and the message delivery has failed. If you receive any of the errors listed below when sending a message to a Mimecast customer, contact the recipient's Mimecast Administrator. Mimecast can only deal with designated customer contacts.”
Administrative prohibition envelope blocked
The sender's email address or domain has triggered a Blocked Senders Policy or there's a SPF hard rejection.
Delete or modify the Blocked Senders policy to exclude the sender address.
Precisely how you “contact the recipient's Mimecast Administrator” when you can’t send an email to Mimecast is indeed a challenge!
How are you sending this email -
from VM Webmail using VM servers
from an email client using VM servers (which client?)
You (and maybe the recipient) will probably want to find out if it is only your IP address (edit to add: sorry - not sure that is possible - it must mean VM's server IP address), your email address specifically, or the domain itself that is being blocked.
I have phoned them and advised but I don't think the person I spoke to considers it an issue to take it up. It is a rather large legal firm and I'm trying to send legal documents. if I could get in touch with their techie staff I might succeed, but the problem is at the recipients end,
I have a gmail account but never use it for Emails. I logged into it and sent the error codes and attachment to the receiver via gmail. It hasn't bounced, I just hope the recipients take it up with their technical people.
I've just started getting the same issue. Mimecast on my work account bounces emails I send from ntlworld.com. Same error - Mimecast error 550. I can see the internal Mimecast error code on my work email, as follows...
"These incoming messages have been blocked by Mimecast for security reasons. No data has been accepted for them and they can't be retrieved.
Rejection Type:13010Rejection Description:Envelope RejectedEvent Time:09/28/2020 at 03:38 PMRejection Information:Policy level block list in force"
I've tried sending from the virgin media web portal and from an iphone, and I've tried to 3 different target email addresses at my work. Mimecast rejects every combo.
This has only just started maybe last week. It does imply Mimecast is blocking ntlworld in its default policy.
As stated in in the above responses, Mimecast will only talk to their customer’s designated Mimecast administrator. So, find out who that is in your company, explain that VM’s NTLworld domain is being blocked and request that the admin asks Mimecast to remove it from their blocklist.
And as I said above, if time is of the essence, the fastest solution may be to open a Gmail account and use that for emails you send to your work addresses.