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The following is an example of how MailMate handles this issue now (and is just for future reference):

11:46:20 C: I18 APPEND INBOX () " 9-Sep-2015 15:21:56 +0200" {9902}  
11:46:20 S: + 9904  
11:46:20 C: ...  
11:46:20    <9902 bytes>  
11:46:20 C: ...  
11:46:20 S: + 7858  
11:46:20 Warning: Unexpected continuation requested (response: “7858”, command: “I18 APPEND INBOX () " 9-Sep-2015 15:21:56 +0200" {9902}”).  
11:46:20 S: + 5812  
11:46:20 Warning: Unexpected continuation requested (response: “5812”, command: “I18 APPEND INBOX () " 9-Sep-2015 15:21:56 +0200" {9902}”).  
11:46:20 S: + 5808  
11:46:20 Warning: Unexpected continuation requested (response: “5808”, command: “I18 APPEND INBOX () " 9-Sep-2015 15:21:56 +0200" {9902}”).  
11:46:20 S: + 3762  
11:46:20 Warning: Unexpected continuation requested (response: “3762”, command: “I18 APPEND INBOX () " 9-Sep-2015 15:21:56 +0200" {9902}”).  
11:46:20 S: + 1716  
11:46:20 Warning: Unexpected continuation requested (response: “1716”, command: “I18 APPEND INBOX () " 9-Sep-2015 15:21:56 +0200" {9902}”).  
11:46:21 S: * 3 EXISTS  
11:46:21 S: I18 OK [APPENDUID 1355825141 321] completed (took 189 ms)

Previously, instead of the warnings MailMate would return an error making the upload fail.

The problem is related to so-called IMAP literals. First, the client can tell the server that more data is coming:

C: TAG Some Command {42}

When the server is ready to receive the 42 bytes then it returns a so-called continuation request:

S: + arbitrary message

And then the client sends the data.

What appears to happen for (some) iCloud servers is that the server receives part of the data and then decides to send another continuation request, and another one, etc.

I believe I know why MailMate could trigger this behavior while it's not triggered by other email clients, but I still don't think it's correct server behavior.

[bulk edit]