when Asynchronous Processing is not used,i found an intersting thing.After 354 Start mail input; end with <CRLF>.<CRLF>

If sender server send: SENT: [nl].

It is 100% no problem

If sender server send: SENT: <\r\n.\r\n>

Most of the case,it has problem,the details looks like below:

(1)

2021-05-22 22:54:00,140 - [     5437] S <-- 354 Start mail input; end with <CRLF>.<CRLF>
2021-05-22 22:55:10,965 - [     5437] S <-- 250 2.6.0 Queued mail for delivery
2021-05-22 22:55:11,137 - [     5437] C --> QUIT
2021-05-22 22:55:11,137 - [     5437] S <-- 221 Good bye. Hope to see you again

(2)

2021-05-22 22:56:15,831 - [     5438] S <-- 354 Start mail input; end with <CRLF>.<CRLF>
2021-05-22 22:57:21,496 - [     5438] S <-- 250 2.6.0 Queued mail for delivery
2021-05-22 22:57:21,496 - [     5438] ~~~~~~~~~~~~ Connection Terminated (66372:515) Unexpected input termination. Connection terminated without a graceful QUIT

Is it a bug? Seems Xeams not understanding \r\n ???

when Asynchronous Processing enabled,never has such problem

After many try,seems it is the network issue? The sender side email log is as below:

[2021-05-22 22:56:22] RECV (SM21052200001477=>admin@mydomain.com): 354 Start mail input; end with <CRLF>.<CRLF>
[2021-05-22 22:56:22] SEND (SM21052200001477=>admin@mydomain.com): <GO...2151974 bytes>
[2021-05-22 22:56:22] SEND (SM21052200001477=>admin@mydomain.com): <\r\n.\r\n>
[2021-05-22 22:57:24] RECV (SM21052200001477=>admin@mydomain.com): network recv failed at [.]
[2021-05-22 22:57:24] SEND (SM21052200001477=>admin@mydomain.com): <FAIL>
[2021-05-22 22:57:24] SEND (SM21052200001477=>admin@mydomain.com): [0] FAILED: network recv failed at [.]
[2021-05-22 22:57:24] SEND (SM21052200001477=>admin@mydomain.com): Sent FAILED to admin@mydomain.com: network recv failed at [.]

[2021-05-22 22:59:27] RECV (SM21052200001477=>admin@mydomain.com): 354 Start mail input; end with <CRLF>.<CRLF>
[2021-05-22 22:59:27] SEND (SM21052200001477=>admin@mydomain.com): <GO...2151974 bytes>
[2021-05-22 22:59:27] SEND (SM21052200001477=>admin@mydomain.com): <\r\n.\r\n>
[2021-05-22 23:00:29] RECV (SM21052200001477=>admin@mydomain.com): true [.]
[2021-05-22 23:00:29] SEND (SM21052200001477=>admin@mydomain.com): <FAIL>
[2021-05-22 23:00:29] SEND (SM21052200001477=>admin@mydomain.com): [0] FAILED: network recv failed at [.]
[2021-05-22 23:00:29] SEND (SM21052200001477=>admin@mydomain.com): Sent FAILED to admin@mydomain.com: network recv failed at [.]

[2021-05-22 23:42:22] RECV (SM21052200001481=>admin@mydomain.com): 354 Start mail input; end with <CRLF>.<CRLF>
[2021-05-22 23:42:22] SEND (SM21052200001481=>admin@mydomain.com): <GO...667743 bytes>
[2021-05-22 23:42:39] SEND (SM21052200001481=>admin@mydomain.com): <\r\n.\r\n>
[2021-05-22 23:42:48] RECV (SM21052200001481=>admin@mydomain.com): 250 2.6.0 Queued mail for delivery
[2021-05-22 23:42:48] SEND (SM21052200001481=>admin@mydomain.com): QUIT
[2021-05-22 23:43:50] RECV (SM21052200001481=>admin@mydomain.com): network recv failed at [QUIT]
[2021-05-22 23:43:50] SEND (SM21052200001481=>admin@mydomain.com): Sent OK to admin@mydomain.com

The problem is solved,most of the email software wait timeout set to be 60~90s by default,change it and that works fine.

You should always use Asynchronous processing in Xeams. Disabling this feature will often result in timeouts. Depending upon the contents of the message, Xeams could take a long time to process an incoming email. When Asynchronous is not used, the sender SMTP server will have to wait until the entire processing is done and will likely timeout.

What is the reason for you not to use Asynchronous?