Here's what happens when I send the message locally with -v.

$ sendmail -v -fuser1@xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s
Subject: another manual test

hi there
^D
atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s... Connecting to [127.0.0.1] via relay...
220 xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s ESMTP Sendmail 8.17.0.2/8.16.1; Fri, 18 Jun 2021 16:56:38 GMT
>>> EHLO xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s
250-xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s Hello localhost [127.0.0.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-SMTPUTF8
250-ETRN
250-STARTTLS
250-DELIVERBY
250 HELP
>>> STARTTLS
220 2.0.0 Ready to start TLS
>>> EHLO xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s
250-xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s Hello localhost [127.0.0.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-SMTPUTF8
250-ETRN
250-DELIVERBY
250 HELP
>>> MAIL From:<user1@xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s> SIZE=39
250 2.1.0 <user1@xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s>... Sender ok
>>> RCPT To:<atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s>
>>> DATA
250 2.1.5 <atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s>... Recipient ok
354 Enter mail, end with "." on a line by itself
>>> .
250 2.0.0 15IGucDw095566 Message accepted for delivery
atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s... Sent (15IGucDw095566 Message accepted for delivery)
Closing connection to [127.0.0.1]
>>> QUIT
221 2.0.0 xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s closing connection

and here's the Received lines when it arrives at my server. In that middle received line it
looks like it turned into EAI:

Received: from xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s ([IPV6:2606:4300:0:4::1003])
by mail1.iecc.com ([IPV6:2001:470:1f07:1126:33:5370:616d:6d31])
with UTF8SMTPS via TCP6 (port 42248/25) id 678221553
tls TLS1_3_ECDHE_RSA_AES_256_GCM_AEAD; 18 Jun 2021 16:56:40 -0000
Received: from xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s (localhost [127.0.0.1])
by xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s (8.17.0.2/8.16.1) with UTF8SMTPS id 15IGucDw095566
(version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO)
for <atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s>; Fri, 18 Jun 2021 16:56:39 GMT
(envelope-from user1@xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s)
Received: (from johnl@localhost)
by xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s (8.17.0.2/8.16.1/Submit) id 15IGuQJX095564
for atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s; Fri, 18 Jun 2021 16:56:26 GMT
(envelope-from user1@xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s)
--
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly

John Levine wrote:
> Here's what happens when I send the message locally with -v.

> $ sendmail -v -fuser1@xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s
> atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s
> Subject: another manual test
>
> hi there

> Received: from xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s (localhost [127.0.0.1])
> by xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s (8.17.0.2/8.16.1)
> with UTF8SMTPS id 15IGucDw095566
> (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO)
> for <atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s>; Fri, 18 Jun 2021
> 16:56:39 GMT
> (envelope-from user1@xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s)

This doesn't happen on my test systems.

Please start the sendmail daemon with -odq, run the test again,
make a copy of the qf/df files for this test message, then run
sendmail -Am -v -qRatest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s 2>&1 |tee log.1
and post log.1, the qf file, and the definition of the Received:
header in your .cf file.

--
Note: please read the netiquette before posting. I will almost never
reply to top-postings which include a full copy of the previous
article(s) at the end because it's annoying, shows that the poster
is too lazy to trim his article, and it's wasting the time of all readers.

According to Claus A�mann <INVALID_NO_CC_REMOVE_IF_YOU_DO_NOT_POST_ml+sendmail(-no-copies-please)@esmtp.org>:
>Please start the sendmail daemon with -odq, run the test again,
>make a copy of the qf/df files for this test message, then run
>sendmail -Am -v -qRatest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s 2>&1 |tee log.1
>and post log.1, the qf file, and the definition of the Received:
>header in your .cf file.

Well, that's strange, this time there's no EAI, and I restarted things a few times, still no EAI.
Will see if I can get it to do what it did before.

I still think you should get rid of the -U flag and decide whether a mesasge needs EAI during the submission
cleanup, like Postfix and Exim and Gmail do.

R's,
John

$ cat log.1
Running /var/spool/mqueue/15J067OQ096648 (sequence 1 of 1)
<atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s>... Connecting to mail1.iecc.com. via esmtp...
220 mail1.iecc.com mailfront ESMTP
>>> EHLO xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s
250-mail1.iecc.com
250-SMTPUTF8
250-AUTH LOGIN
250-LIMITS RCPTMAX=20 MAILMAX=5
250-SIZE 100000000
250-8BITMIME
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 STARTTLS
>>> STARTTLS
220 2.0.0 Ready to start TLS
>>> EHLO xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s
250-mail1.iecc.com
250-SMTPUTF8
250-AUTH LOGIN
250-LIMITS RCPTMAX=20 MAILMAX=5
250-SIZE 100000000
250-8BITMIME
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 STARTTLS
>>> MAIL From:<user1@xn--5nq21jyu9d1ta.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s> SIZE=1146
250 2.1.0 Sender accepted.
>>> RCPT To:<atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s>
>>> DATA
250 2.1.5 Recipient accepted.
354 End your message with a period on a line by itself.
>>> .
250 2.6.0 Accepted message qp 11959 bytes 1798
<atest@xn--fct.xn--5nqx41au4nqohsp3axcg.xn--fiqs8s>... Sent (Accepted message qp 11959 bytes 1798)
Closing connection to mail1.iecc.com.
>>> QUIT
221 2.0.0 Good bye.
--
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly