Looking at the receiving email address [email protected] I suspect you are running into an issue with a recently released feature. On January 16th 2022 we released Plus-Addressing
When you send to any inbox at a mailsac hosted domain, if a + plus symbol is included, we remove that symbol and everything after it. In your case [email protected] is being delivered to [email protected]. Sorry for any issues this has caused you.
Plus-addressing has long been on our backlog and is supported by many large email providers(gmail, fastmail, icloud).