Send Welcome Mail error

[zengateway]

Hello. I get error from smtp server "SMTP protocol synchronization error (next input sent too soon: pipelining was advertised):" when checkbox "send welcome mail" is used.
My SMTP server Exim have "smtp_enforce_sync = true". Is there any way to fix that without disable smtp_enforce_sync?

[DavidGoodwin]

The SMTP sending code isn't particularly elegant / intelligent ....

https://github.com/postfixadmin/postfixadmin/blob/8461afb4ff66419367113ea302316e6873d34390/functions.inc.php#L1539

I don't know what 'smtp_enforce_sync' does, but presumably it's trying to catch spammy clients out due to timing etc?

[zengateway]

smtp_enforce_sync

quote from exim man:

smtp_enforce_sync:

The SMTP protocol specification requires the client to wait for a response from the server at certain points in the dialogue. Without PIPELINING these synchronization points are after every command; with PIPELINING they are fewer, but they still exist.

Some spamming sites send out a complete set of SMTP commands without waiting for any response. Exim protects against this by rejecting a message if the client has sent further input when it should not have. The error response “554 SMTP synchronization error” is sent, and the connection is dropped. Testing for this error cannot be perfect because of transmission delays (unexpected input may be on its way but not yet received when Exim checks). However, it does detect many instances.

[zengateway]

found solution on exim side

"The check can be globally disabled by setting smtp_enforce_sync false. If you want to disable the check selectively (for example, only for certain hosts), you can do so by an appropriate use of a control modifier in an ACL (see section 44.13). See also pipelining_advertise_hosts."