Hi @Glenn Maxwell ,
The sender may receive the following non-delivery report (NDR) that contains the 5.5.0 error code. This code indicates a generic SMTP failure.
Typically, this problem occurs if the destination SMTP server performs a reverse lookup and if one of the following conditions is true:
The IP address does not match the domain name that is used in the return address of the e-mail message.
A pointer (PTR) record does not exist or is not valid for the source SMTP server's IP address.
You may also take a look at the discussion here: smtp error 5.5.0 544
That is an error being generated by your ISP. It isn't an Exchange error. You will need to contact them to establish whether they can support additional domains through their smart host.
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