From what I have read, by a link provided in the thread, the 2 supported fixes are either add support for BDAT or Disable bare line feed rejection (allow messages that contain bare line feeds) in the destination email server.
It seems like this is going to be an increasingly common issue. This is from Microsoft's website linked in the post
DKIM is being pushed more and more and if that is part of what triggers the error code 550 5.6.11 then it would be beneficial to support this feature.Microsoft 365 and Office 365 used to remove bare line feeds from messages to enable delivery to older email servers that didn't support SMTP Chunking and the BDAT command. In an effort to better support security standards (for example, DomainKeys Identified Mail or DKIM), Office 365 no longer removes bare line feeds from messages.
Thank you for the awesome product, it has served me well for many years and would like that to continue.