I've been playing with the PHP verifyEmail class. Worked quite nice until I started using it for a large website with lots of validations. It seems there are now websites that do not accept this kind of validation anymore, where the 'rcpt to:' command throws a 550 5.5.1 Protocol error. The requested user is actually valid for that mail server.
This can be tested with the following:
telnet mx.transip.email 25
Connected to mx.transip.email.
Escape character is '^]'.
mail from: <firstname.lastname@example.org>
250 2.1.0 Ok
rcpt to: <email@example.com>
550 5.5.1 Protocol error
Did you encounter this behaviour before, is there already a fix for it? Main issue is that 550 is of course that 550 is the same error message as unknown user besides the 5.5.1 vs 5.1.1 subcode.