Hi,
The recent introduction of mail address length verification is causing us to lose messages sent to mailpit via sendgrid. Sendgrid generates an overly long FROM address which goes over the 63 byte local part limit.
I am not sure though that mailpit is responding correctly in this case. The RFC suggests (https://datatracker.ietf.org/doc/html/rfc5321#section-4.5.3.1.9), that the correct response in this case is probably 500 Line too long., where mailpit is responding 501 5.5.4 Syntax error in parameters or arguments (invalid FROM parameter)".
I am not sure if sendgrid would re-send if presented with the correct message, but it seems plausible.