I'm having the same problem and am wondering if Google's two-level authentication is casing the problem. Here's the error message I get:
Starting job: Email TheAtlantic.com [Mon, 20 Dec 2021] to
ipa.stout@free.kindle.com
connect: ('smtp.gmail.com', 465)
connect: ('smtp.gmail.com', 465)
reply: b'220 smtp.gmail.com ESMTP e15sm238366qtq.83 - gsmtp\r\n'
reply: retcode (220); Msg: b'smtp.gmail.com ESMTP e15sm238366qtq.83 - gsmtp'
connect: b'smtp.gmail.com ESMTP e15sm238366qtq.83 - gsmtp'
send: 'ehlo DESKTOP-LQKFKS4.nycap.rr.com\r\n'
reply: b'250-smtp.gmail.com at your service, [2603:7080:306:665:20ef:3136:a694:ed3]\r\n'
reply: b'250-SIZE 35882577\r\n'
reply: b'250-8BITMIME\r\n'
reply: b'250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH\r\n'
reply: b'250-ENHANCEDSTATUSCODES\r\n'
reply: b'250-PIPELINING\r\n'
reply: b'250-CHUNKING\r\n'
reply: b'250 SMTPUTF8\r\n'
reply: retcode (250); Msg: b'smtp.gmail.com at your service, [2603:7080:306:665:20ef:3136:a694:ed3]\nSIZE 35882577\n8BITMIME\nAUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH\nENHANCEDSTATUSCODES\nPIPELINING\nCHUNKING\ nSMTPUTF8'
send: AUTH <censored>
reply: b'534-5.7.9 Application-specific password required. Learn more at\r\n'
reply: b'534 5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor e15sm238366qtq.83 - gsmtp\r\n'
reply: retcode (534); Msg: b'5.7.9 Application-specific password required. Learn more at\n5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor e15sm238366qtq.83 - gsmtp'
send: AUTH <censored>
reply: b'334 UGFzc3dvcmQ6\r\n'
reply: retcode (334); Msg: b'UGFzc3dvcmQ6'
send: 'V2F0ZXI5OW1lbG9u\r\n'
reply: b'534-5.7.9 Application-specific password required. Learn more at\r\n'
reply: b'534 5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor e15sm238366qtq.83 - gsmtp\r\n'
reply: retcode (534); Msg: b'5.7.9 Application-specific password required. Learn more at\n5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor e15sm238366qtq.83 - gsmtp'
Sending failed...
Traceback (most recent call last):
File "calibre\gui2\email.py", line 49, in run
File "calibre\gui2\email.py", line 133, in sendmail
File "calibre\utils\smtp.py", line 180, in sendmail
File "smtplib.py", line 734, in login
File "smtplib.py", line 723, in login
File "smtplib.py", line 646, in auth
smtplib.SMTPAuthenticationError: (534, b'5.7.9 Application-specific password required. Learn more at\n5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor e15sm238366qtq.83 - gsmtp')
Retrying in 301 seconds...
Waiting 301 seconds before sending, to avoid being marked as spam.
You can control this delay via Preferences->Tweaks
connect: ('smtp.gmail.com', 465)
connect: ('smtp.gmail.com', 465)
reply: b'220 smtp.gmail.com ESMTP m1sm17428938qtk.34 - gsmtp\r\n'
reply: retcode (220); Msg: b'smtp.gmail.com ESMTP m1sm17428938qtk.34 - gsmtp'
connect: b'smtp.gmail.com ESMTP m1sm17428938qtk.34 - gsmtp'
send: 'ehlo DESKTOP-LQKFKS4.nycap.rr.com\r\n'
reply: b'250-smtp.gmail.com at your service, [2603:7080:306:665:20ef:3136:a694:ed3]\r\n'
reply: b'250-SIZE 35882577\r\n'
reply: b'250-8BITMIME\r\n'
reply: b'250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH\r\n'
reply: b'250-ENHANCEDSTATUSCODES\r\n'
reply: b'250-PIPELINING\r\n'
reply: b'250-CHUNKING\r\n'
reply: b'250 SMTPUTF8\r\n'
reply: retcode (250); Msg: b'smtp.gmail.com at your service, [2603:7080:306:665:20ef:3136:a694:ed3]\nSIZE 35882577\n8BITMIME\nAUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH\nENHANCEDSTATUSCODES\nPIPELINING\nCHUNKING\ nSMTPUTF8'
send: AUTH <censored>
reply: b'534-5.7.9 Application-specific password required. Learn more at\r\n'
reply: b'534 5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor m1sm17428938qtk.34 - gsmtp\r\n'
reply: retcode (534); Msg: b'5.7.9 Application-specific password required. Learn more at\n5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor m1sm17428938qtk.34 - gsmtp'
send: AUTH <censored>
reply: b'334 UGFzc3dvcmQ6\r\n'
reply: retcode (334); Msg: b'UGFzc3dvcmQ6'
send: 'V2F0ZXI5OW1lbG9u\r\n'
reply: b'534-5.7.9 Application-specific password required. Learn more at\r\n'
reply: b'534 5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor m1sm17428938qtk.34 - gsmtp\r\n'
reply: retcode (534); Msg: b'5.7.9 Application-specific password required. Learn more at\n5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor m1sm17428938qtk.34 - gsmtp'
Sending failed...
Traceback (most recent call last):
File "calibre\gui2\email.py", line 49, in run
File "calibre\gui2\email.py", line 133, in sendmail
File "calibre\utils\smtp.py", line 180, in sendmail
File "smtplib.py", line 734, in login
File "smtplib.py", line 723, in login
File "smtplib.py", line 646, in auth
smtplib.SMTPAuthenticationError: (534, b'5.7.9 Application-specific password required. Learn more at\n5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor m1sm17428938qtk.34 - gsmtp')
Job: "Email TheAtlantic.com [Mon, 20 Dec 2021] to
ipa.stout@free.kindle.com" failed with error:
Traceback (most recent call last):
File "calibre\gui2\threaded_jobs.py", line 83, in start_work
File "calibre\gui2\email.py", line 106, in __call__
File "calibre\gui2\email.py", line 49, in run
File "calibre\gui2\email.py", line 133, in sendmail
File "calibre\utils\smtp.py", line 180, in sendmail
File "smtplib.py", line 734, in login
File "smtplib.py", line 723, in login
File "smtplib.py", line 646, in auth
smtplib.SMTPAuthenticationError: (534, b'5.7.9 Application-specific password required. Learn more at\n5.7.9
https://support.google.com/mail/?p=InvalidSecondFactor m1sm17428938qtk.34 - gsmtp')
Called with args: ('C:\\Users\\Charlie\\AppData\\Local\\Temp\\calibr e_kyv57rgq\\bjrrgmfccaltmpfmt.MOBI', 'TheAtlantic.com [Mon, 20 Dec 2021] - calibre.MOBI', 'ipa.stout@free.kindle.com', 'E-book: TheAtlantic.com [Mon, 20 Dec 2021]', 'Attached, you will find the e-book\n\nTheAtlantic.com [Mon, 20 Dec 2021]\n\tby calibre\n\nin the MOBI format.') {'notifications': <queue.Queue object at 0x000002E10B17D160>, 'abort': <threading.Event object at 0x000002E10B17DC40>, 'log': <calibre.utils.logging.GUILog object at 0x000002E10B17D7C0>}