Freron MailMate

We use Office 365 at the workplace. Mailmate fails to send an email when my recipient's name (full name, not email address) contains a French accent aigu (é). This is reproducible: it happens every time, and when I edit out the accent aigu and replace the character with a normal e, the mail is accepted. Trying with Outlook and Thunderbird, the issue does not occur there (I also have reports from a mutt/postfix configuration that works).

I rewrite the recipient's name with X's in the SMTP dialogue I fetched from the activity viewer. It seems the SMTP encoding is set to 8BITMIME and then SMTPUTF8, and the remote side chokes on the accent. This may or may not be an issue on Mailmate's side - knowing Outlook, it could be on their side, but in that case Outlook and Thunderbird somehow work around it.

11:13:23 S: 220 AM0PR08CA0013.outlook.office365.com Microsoft ESMTP MAIL Service ready at Mon, 16 May 2022 11:13:23 +0000
11:13:23 C: EHLO [130.89.226.112]
11:13:23 S: 250-AM0PR08CA0013.outlook.office365.com Hello [130.89.226.112]
11:13:23 S: 250-SIZE 157286400
11:13:23 S: 250-PIPELINING
11:13:23 S: 250-DSN
11:13:23 S: 250-ENHANCEDSTATUSCODES
11:13:23 S: 250-STARTTLS
11:13:23 S: 250-8BITMIME
11:13:23 S: 250-BINARYMIME
11:13:23 S: 250-CHUNKING
11:13:23 S: 250 SMTPUTF8
11:13:23 C: STARTTLS
11:13:23 S: 220 2.0.0 SMTP server ready
11:13:23 Initiating secure connection...
11:13:23 Prepare secure connection...
11:13:23 Returned (4)...
11:13:23 Protocol version: kTLSProtocol12
11:13:23 C: EHLO [130.89.226.112]
11:13:23 S: 250-AM0PR08CA0013.outlook.office365.com Hello [130.89.226.112]
11:13:23 S: 250-SIZE 157286400
11:13:23 S: 250-PIPELINING
11:13:23 S: 250-DSN
11:13:23 S: 250-ENHANCEDSTATUSCODES
11:13:23 S: 250-AUTH LOGIN XOAUTH2
11:13:23 S: 250-8BITMIME
11:13:23 S: 250-BINARYMIME
11:13:23 S: 250-CHUNKING
11:13:23 S: 250 SMTPUTF8
11:13:23 Retrieving password (keychain or user request)
11:13:23 C: AUTH XOAUTH2 ••••••••••
11:13:23 S: 235 2.7.0 Authentication successful
11:13:23 C: MAIL FROM:<...> SIZE=4426
11:13:23 S: 250 2.1.0 Sender OK
11:13:23 C: RCPT TO:<...>
11:13:23 S: 250 2.1.5 Recipient OK
11:13:23 C: DATA
11:13:23 S: 354 Start mail input; end with .
11:13:23 C: From: Ralph Holz <...>
11:13:23 C: To: XXXXXXXX, =?utf-8?q?XXXX=C3=A9?= <... ...
11:13:23 <4429 bytes>
... 11:13:23 C: .
11:13:23 S: 554 5.2.0 STOREDRV.Submission.Exception:InvalidRecipientsException; Failed to process message due to a permanent exception with message [BeginDiagnosticData]Recipient 'XXXXXXXX' is not resolved. All recipients must be resolved before a message can be submitted. InvalidRecipientsException: Recipient 'XXXXXXXX' is not resolved. All recipients must be resolved before a message can be submitted.[EndDiagnosticData] [Hostname=AM8P195MB1092.EURP195.PROD.OUTLOOK.COM]
11:13:23 Error: Unexpected return code 554 (expected 250):
“5.2.0 STOREDRV.Submission.Exception:InvalidRecipientsException; Failed to process message due to a permanent exception with message [BeginDiagnosticData]Recipient 'XXXXXXXX' is not resolved. All recipients must be resolved before a message can be submitted. InvalidRecipientsException: Recipient 'XXXXXXXX' is not resolved. All recipients must be resolved before a message can be submitted.[EndDiagnosticData] [Hostname=AM8P195MB1092.EURP195.PROD.OUTLOOK.COM]”. 11:13:23 Error code: 2000
11:13:23 Failed action (0). Reset observed read/write timeouts: 8/8

11:13:23 Handling reply
11:13:54 Handling request
11:13:54 C: QUIT
11:13:54 S: 221 2.0.0 Service closing transmission channel
11:13:54 Clearing connection to smtp.office365.com
11:13:54 Ready to run action (type: 3, retry count: 0)
11:13:54 Clearing connection to smtp.office365.com
11:13:54 Completed action (3). Observed read/write timeouts: 8/8

11:13:54 Handling reply