SMTP: this message has been in the queue too long

[stars]

Hello there,
I've installed ISP Config 3 on two servers (multiserver setup) - everything works fine except SMTP.

1) I can send emails from one local account to another local account (using webmail or external mail client)
2) I can send email from ISP to outside servers (using both webmail or external mail client)

but 3) I can't sent from outside servers to ispconfig server - after while I get failure notice: (#4.4.2) I'm not going to try again; this message has been in the queue too long.

Does anybody know what could be wrong? Any ideas?

regards
Stars

[till]

Check your mail log for errors.

[stars]

After few weeks od testing we figured out that port 25 was open for outgoing connection, was open for incomming connection but only for users of local LAN. Connection to port 25 from outside networks was blocked. It was caused by internet service providers router with bad configuration. Now it is fixed.
thanks!
Stars