Big Sur smtpclient.apple connection to smtp.gmail.com reports failure on Mail Connection Doctor

The Mail Connection Doctor reports "Trying to log into SMTP account failed." Review of the log (below) suggests the "501 5.5.2 Cannot Decode response" is caused by the * (unexpected) cancel(?) issued at 12:04:23.704 which I assume leads to the failure status!

Is the "Check Again" simply trying to end the session because there is no actual email to be sent?

Curiously other (postfix) servers I connect to do not use the same command sequence and this doesn't seem to impede sending outbound emails via gmail.

Is there a configuration setting that I might be missing? Cheers, Frank.

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INITIATING CONNECTION Jan 04 12:04:21.876 host:smtp.gmail.com -- port:587 -- socket:0x0 -- thread:0x6000052b1040

CONNECTED Jan 04 12:04:21.991 [kCFStreamSocketSecurityLevelNone] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

READ Jan 04 12:04:22.286 [kCFStreamSocketSecurityLevelNone] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

220 smtp.gmail.com ESMTP a15-20020a63e84f000000b0043b565cb57csm19136637pgk.73 - gsmtp

WROTE Jan 04 12:04:22.286 [kCFStreamSocketSecurityLevelNone] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

EHLO smtpclient.apple

READ Jan 04 12:04:22.562 [kCFStreamSocketSecurityLevelNone] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

250-smtp.gmail.com at your service, [123.208.20.153]

250-SIZE 35882577

250-8BITMIME

250-STARTTLS

250-ENHANCEDSTATUSCODES

250-PIPELINING

250-CHUNKING

250 SMTPUTF8

WROTE Jan 04 12:04:22.562 [kCFStreamSocketSecurityLevelNone] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

STARTTLS

READ Jan 04 12:04:22.829 [kCFStreamSocketSecurityLevelNone] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

220 2.0.0 Ready to start TLS

WROTE Jan 04 12:04:23.168 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

EHLO smtpclient.apple

READ Jan 04 12:04:23.435 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

250-smtp.gmail.com at your service, [123.208.20.153]

250-SIZE 35882577

250-8BITMIME

250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH

250-ENHANCEDSTATUSCODES

250-PIPELINING

250-CHUNKING

250 SMTPUTF8

WROTE Jan 04 12:04:23.436 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

AUTH LOGIN

READ Jan 04 12:04:23.704 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

334 VXxxxxxxxxU6

WROTE Jan 04 12:04:23.704 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

*

READ Jan 04 12:04:23.974 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

501 5.5.2 Cannot Decode response a15-20020a63e84f000000b0043b565cb57csm19136637pgk.73 - gsmtp

WROTE Jan 04 12:04:23.974 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

AUTH LOGIN

READ Jan 04 12:04:24.247 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

334 VXxxxxxxxxU6

WROTE Jan 04 12:04:24.247 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

*

READ Jan 04 12:04:24.519 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

501 5.5.2 Cannot Decode response a15-20020a63e84f000000b0043b565cb57csm19136637pgk.73 - gsmtp

WROTE Jan 04 12:04:24.519 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:smtp.gmail.com -- port:587 -- socket:0x60000629dc20 -- thread:0x6000052b1040

QUIT