dovecot warns (non-fatal) "invalid EHLO response line: Unexpected character in EHLO keyword" connecting to submission relay ?

I'm setting up an alternative submission relay host for dovecot. Atm, it's pointing @ fastmail.com.

with dovecot config,

submission_relay_host       = smtp.fastmail.com
submission_relay_port       = 465
submission_relay_ssl        = smtps
submission_relay_ssl_verify = no
submission_relay_trusted    = yes
submission_relay_user       = 'acctID@mydomain.com'
submission_relay_password   = 'acctPWD'

submission relay via dovecot proxy works as expected -- mail's delivered/received by intended recipient.

in local dovecot logs I notice

==> /var/log/dovecot/dovecot.log <==
2020-09-24 16:22:19 submission(recipient@example.com)<xxxxxxxxxxxxxxxx>: Warning: smtp-client: conn smtp.fastmail.com:465 (66.111.4.140:465) [1]: Received invalid EHLO response line: Unexpected character in EHLO keyword

at debug level

==> /var/log/dovecot/dovecot-debug.log <==
2020-09-24 16:22:19 submission(recipient@example.com)<xxxxxxxxxxxxxxxx>: Debug: smtp-client: conn smtp.fastmail.com:465 (66.111.4.140:465) [1]: command EHLO: Got reply (1/1): 250 mail.messagingengine.com PIPELINING SIZE 73000000 AUTH DIGEST-MD5 CRAM-MD5 NTLM LOGIN PLAIN AUTH=DIGEST-MD5 CRAM-MD5 NTLM LOGIN PLAIN ENHANCEDSTATUSCODES 8BITMIME (1 commands pending, 0 commands queued)

it's clearly non-fatal, simply a warning.

i certainly do _not_ see this with every relay host, so suspect it's unique to fastmail's EHLO response. at least within the set of hosts I've tested.

i'd like to find/fix it ... either at my dovecot instance, or @ fastmail.

what's specifically invalid about

250 mail.messagingengine.com PIPELINING SIZE 73000000 AUTH DIGEST-MD5 CRAM-MD5 NTLM LOGIN PLAIN AUTH=DIGEST-MD5 CRAM-MD5 NTLM LOGIN PLAIN ENHANCEDSTATUSCODES 8BITMIME

by dovecot's 'measure'?