When the Internet Mail Connector is stopped, it allows up to 60 seconds for any pending message deliveries to complete before shutting down. Any messages that are not delivered within 60 seconds will be retried at the next connection. Dial-up connections are of a fixed duration, so a message that takes longer than that amount of time to transfer will never be delivered - the sending host would keep retrying delivery until it eventually was returned as undeliverable.

With a dial-up connection you should consider setting a message size limit at the Internet Mail Connector. You should choose the message size limit and connection duration such that a message within the size limit can be delivered during a single connection. The size limit will prevent the Internet Mail Connector from trying to deliver inbound or outbound messages larger than the specified size. Users in your site will be prevented from sending large outgoing messages, but this will not protect against large incoming messages, since they must be transmitted over the connection before the Internet Mail Connector can reject them. Check with your service provider to find out if they can limit the sizes of messages they attempt to deliver to your domain.

How do I make sure the Internet Mail Connector doesn't time-out when sending messages over my low-bandwidth dial-up connection?

There are several registry values under the Internet Mail Connector Parameters key
(HKEY_LOCAL_MACHINE\ System\CurrentControlSet\
Services\MsExchangeIMC\Parameters) that control
time-outs:

SMTPRecvTimeout   -  waiting for packet transmission

SMTPWaitForAck    - waiting for OK response to a command sent to another host

SMTPWaitForBanner  -  waiting for HELO banner from another SMTP host

SMTPWaitForDataBlock  - waiting for the termination to a DATA block

SMTPWaitForDataInitiation -  waiting for the start of transfer of a data block

SMTPWaitForDataTermination - waiting for a response to the termination of a data block

SMTPWaitForMailFrom -   waiting for a response to the FROM command

SMTPWaitForRcpt  -   waiting for a response to the RCPT command

These time-outs are specified in seconds. If a time-out occurs, an event will be written to the Windows NT event log. If these time-outs occur frequently, you may want to increase them. In particular, the default setting for SMTPWaitForDataBlock limits the data transfer for a message to 10 minutes. Over a low bandwidth line you may want to increase this value to allow more time for large messages to transmit.

What can I do to optimize the Internet Mail Connector's performance over a low-bandwidth connection?

Using the Microsoft Exchange Server Administrator application, open the Internet Mail Connector properties in the Connections container. The Advanced Transfer Mode options on the Connections page include settings for the maximum number of inbound and outbound connections, and for the maximum number of messages sent in a connection. The default settings are optimized for connections at LAN speeds. For a dial-up connection, you should set the maximum number of inbound and outbound connections to one each. You should increase the maximum number of messages sent in a single connection to a higher value, such as 50.

Performance over a low-bandwidth connection will also be improved if you configure the Internet Mail Connector to forward all mail to your service provider's SMTP host, and let it perform the DNS resolutions to deliver mail to the appropriate destinations.