Mailman option RCPT_BASE64_HEADER_NAME causes TypeError

Symptoms

List members didn't receive emails. Emails failed with the following error.

File "/usr/local/cpanel/3rdparty/mailman/Mailman/Handlers/SMTPDirect.py", 
line 379, in verpdeliver
msgcopy[mm_cfg.RCPT_BASE64_HEADER_NAME] = b64encode(recip)
File "/usr/lib64/python3.6/base64.py", line 58, in b64encode
encoded = binascii.b2a_base64(s, newline=False)
TypeError: a bytes-like object is required, not 'str'

Description

Mailman has an option "RCPT_BASE64_HEADER_NAME," which allows a defined header name to be added to personalized and VERPed deliveries. However, upgrading Mailman's Python 2 to Python 3 has caused emails to become shunted when using "RCPT_BASE64_HEADER_NAME." List members won't receive the delivery due to a failure to handle the message headers.

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-47503. Follow this article to receive an email notification when a solution is published in the product.

Workaround

Disable RCPT_BASE64_HEADER_NAME from inside of mm.cfg, for example:

/usr/local/cpanel/3rdparty/mailman/Mailman/mm_cfg.py:
RCPT_BASE64_HEADER_NAME = ''

Then restart mailman.

/scripts/restartsrv_mailman