MD5 is used by default by upstream for backward compatibility. I think we can downstream patch it to use something better in our default config like sha1. Changing it is simple:

# postconf -e smtpd_tls_fingerprint_digest=sha1
# systemctl restart postfix

  Looks like a related issue:

Core was generated by `smtp -t unix -u'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x0000000000000000 in ?? ()
(gdb) bt
#0  0x0000000000000000 in ?? ()
#1  0x00007f1fa40e9bf7 in tls_data_fprint (buf=buf@entry=0x5648a5948920 "0\202\006G0\202\004/\240\003\002\001\002\002\002>\a0\r\006\t*\206H\206\367\r\001\001\v\005", len=len@entry=1611, mdalg=mdalg@entry=0x5648a585c430 "md5")
    at tls_fprint.c:299
#2  0x00007f1fa40e9ce3 in tls_cert_fprint (peercert=peercert@entry=0x5648a594afb0, mdalg=0x5648a585c430 "md5") at tls_fprint.c:323
#3  0x00007f1fa40ef9b8 in verify_extract_print (props=0x7ffffd63b120, props=0x7ffffd63b120, peercert=0x5648a594afb0, TLScontext=0x5648a592fd20) at tls_client.c:1097
#4  tls_client_start (props=props@entry=0x7ffffd63b120) at tls_client.c:1097
#5  0x00005648a39b295b in smtp_start_tls (state=state@entry=0x5648a592a8f0) at smtp_proto.c:910
#6  0x00005648a39b24ad in smtp_helo (state=state@entry=0x5648a592a8f0) at smtp_proto.c:790
#7  0x00005648a39ae428 in smtp_connect_inet (state=state@entry=0x5648a592a8f0, nexthop=nexthop@entry=0x5648a58e1ca0 "[<<<example.com>>>]", def_service=0x5648a585d280 "smtp") at smtp_connect.c:966
#8  0x00005648a39af033 in smtp_connect (state=0x5648a592a8f0) at smtp_connect.c:1169
#9  0x00005648a39acf3c in deliver_message (request=0x5648a592f820, service=0x7ffffd63bf8c "smtp") at smtp.c:1029
#10 smtp_service (client_stream=0x5648a586acb0, service=0x7ffffd63bf8c "smtp", argv=<optimized out>) at smtp.c:1061
#11 0x00007f1fa42fcd3a in single_server_wakeup (fd=<optimized out>, attr=0x0) at single_server.c:287
#12 0x00007f1fa3a64788 in event_loop (delay=<optimized out>) at events.c:1186
#13 0x00007f1fa42fdd7a in single_server_main (argc=<optimized out>, argv=<optimized out>, service=0x5648a39acea0 <smtp_service>) at single_server.c:790
#14 0x00005648a39acd50 in main (argc=4, argv=0x7ffffd63bd68) at ../../include/mail_server.h:87
(gdb) f 1
#1  0x00007f1fa40e9bf7 in tls_data_fprint (buf=buf@entry=0x5648a5948920 "0\202\006G0\202\004/\240\003\002\001\002\002\002>\a0\r\006\t*\206H\206\367\r\001\001\v\005", len=len@entry=1611, mdalg=mdalg@entry=0x5648a585c430 "md5")
    at tls_fprint.c:299
299	    digest_data(buf, len);
(gdb) p mdctx
$1 = (EVP_MD_CTX *) 0x5648a5938550
(gdb) p *mdctx
$2 = {digest = 0x0, engine = 0x0, flags = 0, md_data = 0x0, pctx = 0x0, update = 0x0}

  Any suggestion about the issue?

# rpm -q postfix
postfix-3.3.1-12.el8.x86_64
# cat /etc/redhat-release 
Red Hat Enterprise Linux release 8.2 (Ootpa)

From the upstream documentation:

The default algorithm is sha256 with Postfix ≥ 3.6 and the compatibility_level set to 3 or higher. With Postfix ≤ 3.5, the default algorithm is md5.

The best-practice algorithm is now sha256. Recent advances in hash function cryptanalysis have led to md5 and sha1 being deprecated in favor of sha256. However, as long as there are no known "second pre-image" attacks against the older algorithms, their use in this context, though not recommended, is still likely safe.

The postfix-3.6 is not yet stable.

Changing the hash to sha256 mid-release could break several customers, i.e. change of the hash will require updating any associated lookup table keys with the "sha256" digests of the expected client certificate or public key. This is something we cannot do automatically.

It's risky to change the defaults during lifetime of the product (see comment 10 and comment 11), we will document it in the KB article and change the default settings in the next major RHEL release, so closing as wontfix.