What causes strange FolderClosedException on fetching mail via IMAP and how to resolve it? - jakarta-mail

Question

Fetching some (not all) mails in a folder, I encounter strange FolderClosedException after almost excactly two minutes. It looks like the connection freezes after calling MimeBodyPart::getContent. I suppose it is thrown because of some timeout on the server side.
The affected mails are automatically forwarded from an Office365 mailbox to this IMAP server and have the same MIME structure. I can not tell what server this is.
My runtime is a JakartaEE 8 web app hosted in WildFly 17.0.1.Final running with JRE 11.0.7+10.
No special code. It just loops through all mails in the folder, checks flags and subject, processes MIME structure and downloads relevant attachments.
try (Store store = session.getStore();) {
store.connect(user, password);
Folder folder = store.getFolder(folderName);
...
folder.open(Folder.READ_WRITE);
try {
for (Message message : folder.getMessages()) {
if (message.isSet(Flags.Flag.SEEN)) {
continue;
}
message.getSubject();
...
Object content = message.getContent();
if (content instanceof Multipart) {
...
Multipart multipart = (Multipart) content;
for (int index = 0; index < multipart.getCount(); ++index) {
...
Object partContent = multipart.getBodyPart(index).getContent();
}
}
}
} finally {
folder.close(false);
}
}
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG: JavaMail version 1.6.2
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG: successfully loaded resource: /META-INF/javamail.default.address.map
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG: getProvider() returning javax.mail.Provider[STORE,imaps,com.sun.mail.imap.IMAPSSLStore,Oracle]
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG IMAPS: mail.imap.fetchsize: 16384
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG IMAPS: mail.imap.ignorebodystructuresize: false
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG IMAPS: mail.imap.statuscachetimeout: 1000
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG IMAPS: mail.imap.appendbuffersize: -1
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG IMAPS: mail.imap.minidletime: 10
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG IMAPS: closeFoldersOnStoreFailure
2023-01-02 14:15:10,079 INFO [stdout] (EJB default - 8) DEBUG IMAPS: trying to connect to host "XXX", port 993, isSSL true
2023-01-02 14:15:10,220 INFO [stdout] (EJB default - 8) * OK [CAPABILITY IMAP4rev1 CHILDREN ENABLE ID IDLE LIST-EXTENDED LIST-STATUS LITERAL- MOVE NAMESPACE QUOTA SASL-IR SORT SPECIAL-USE THREAD=ORDEREDSUBJECT UIDPLUS UNSELECT WITHIN AUTH=LOGIN AUTH=PLAIN] IMAP server ready XXX
2023-01-02 14:15:10,220 INFO [stdout] (EJB default - 8) DEBUG IMAPS: AUTH: LOGIN
2023-01-02 14:15:10,220 INFO [stdout] (EJB default - 8) DEBUG IMAPS: AUTH: PLAIN
2023-01-02 14:15:10,220 INFO [stdout] (EJB default - 8) DEBUG IMAPS: protocolConnect login, host=XXX, user=XXX, password=<non-null>
2023-01-02 14:15:10,220 INFO [stdout] (EJB default - 8) DEBUG IMAPS: AUTHENTICATE PLAIN command trace suppressed
2023-01-02 14:15:10,298 INFO [stdout] (EJB default - 8) DEBUG IMAPS: AUTHENTICATE PLAIN command result: BJF0 OK AUTHENTICATE completed
2023-01-02 14:15:10,298 INFO [stdout] (EJB default - 8) BJF1 CAPABILITY
2023-01-02 14:15:10,314 INFO [stdout] (EJB default - 8) * CAPABILITY IMAP4rev1 CHILDREN ENABLE ID IDLE LIST-EXTENDED LIST-STATUS LITERAL- MOVE NAMESPACE QUOTA SASL-IR SORT SPECIAL-USE THREAD=ORDEREDSUBJECT UIDPLUS UNSELECT WITHIN APPENDLIMIT=140000000
2023-01-02 14:15:10,376 INFO [stdout] (EJB default - 8) * CAPABILITY IMAP4rev1 CHILDREN ENABLE ID IDLE LIST-EXTENDED LIST-STATUS LITERAL- MOVE NAMESPACE QUOTA SASL-IR SORT SPECIAL-USE THREAD=ORDEREDSUBJECT UIDPLUS UNSELECT WITHIN APPENDLIMIT=140000000
2023-01-02 14:15:10,376 INFO [stdout] (EJB default - 8) BJF1 OK CAPABILITY completed
2023-01-02 14:15:10,376 INFO [stdout] (EJB default - 8) DEBUG IMAPS: connection available -- size: 1
2023-01-02 14:15:10,376 INFO [stdout] (EJB default - 8) BJF2 SELECT INBOX
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) * 45 EXISTS
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) * 1 RECENT
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) * FLAGS (\Answered \Flagged \Deleted \Seen \Draft $Forwarded)
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) * OK [PERMANENTFLAGS (\Answered \Flagged \Draft \Deleted \Seen $Forwarded \*)] Unlimited
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) * OK [UNSEEN 45] Message 45 is first unseen
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) * OK [UIDNEXT 50] Predicted next UID
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) * OK [UIDVALIDITY 1667372338] UIDs valid
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) BJF2 OK [READ-WRITE] SELECT completed
2023-01-02 14:15:10,392 INFO [stdout] (EJB default - 8) BJF3 FETCH 1 (FLAGS)
2023-01-02 14:15:10,408 INFO [stdout] (EJB default - 8) * 1 FETCH (FLAGS (\Seen))
2023-01-02 14:15:10,408 INFO [stdout] (EJB default - 8) BJF3 OK FETCH completed
.
.
.
2023-01-02 14:15:11,236 INFO [stdout] (EJB default - 8) BJF47 FETCH 45 (FLAGS)
2023-01-02 14:15:11,251 INFO [stdout] (EJB default - 8) * 45 FETCH (FLAGS (\Recent))
2023-01-02 14:15:11,251 INFO [stdout] (EJB default - 8) BJF47 OK FETCH completed
2023-01-02 14:15:11,251 INFO [stdout] (EJB default - 8) BJF48 FETCH 45 (ENVELOPE INTERNALDATE RFC822.SIZE)
2023-01-02 14:15:11,329 INFO [stdout] (EJB default - 8) * 45 FETCH (RFC822.SIZE 12511 INTERNALDATE "02-Jan-2023 13:12:56 +0000" ENVELOPE ("Mon, 2 Jan 2023 14:12:51 +0100 (CET)" {102}
2023-01-02 14:15:11,329 INFO [stdout] (EJB default - 8) =?UTF-8?Q?XXX?=
2023-01-02 14:15:11,329 INFO [stdout] (EJB default - 8) =?UTF-8?Q?XXX?= ((NIL NIL "XXX" "XXX")) ((NIL NIL "XXX" "XXX")) ((NIL NIL "XXX" "XXX")) ((NIL NIL "XXX" "XXX")(NIL NIL "XXX" "XXX")) ((NIL NIL "XXX" "XXX")(NIL NIL "XXX" "XXX")) NIL NIL "XXX"))
2023-01-02 14:15:11,392 INFO [stdout] (EJB default - 8) BJF48 OK FETCH completed
2023-01-02 14:15:11,392 INFO [stdout] (EJB default - 8) BJF49 FETCH 45 (BODYSTRUCTURE)
2023-01-02 14:15:11,408 INFO [stdout] (EJB default - 8) * 45 FETCH (BODYSTRUCTURE (("TEXT" "plain" ("charset" "us-ascii") NIL NIL "7BIT" 87 1 NIL NIL NIL NIL)("APPLICATION" "octet-stream" ("name*" "UTF-8''XXX") NIL NIL "BASE64" 2524 NIL ("attachment" ("filename*" "UTF-8''XXX")) NIL NIL) "mixed" ("boundary" "----=_Part_6_175725035.1672665171185") NIL NIL NIL))
2023-01-02 14:15:11,470 INFO [stdout] (EJB default - 8) BJF49 OK FETCH completed
2023-01-02 14:15:11,470 INFO [stdout] (EJB default - 8) BJF50 FETCH 45 (BODY[1]<0.87>)
2023-01-02 14:17:11,664 ERROR [stderr] (EJB default - 8) javax.mail.FolderClosedException
2023-01-02 14:17:11,664 ERROR [stderr] (EJB default - 8) at javax.mail.api#1.6.2//javax.mail.internet.MimeBodyPart.getContent(MimeBodyPart.java:685)
.
.
.
2023-01-02 14:17:11,696 ERROR [stderr] (EJB default - 8) at org.jboss.threads#2.3.3.Final//org.jboss.threads.JBossThread.run(JBossThread.java:485)
2023-01-02 14:17:11,696 INFO [stdout] (EJB default - 8) DEBUG IMAPS: IMAPStore cleanup, force false
2023-01-02 14:17:11,696 INFO [stdout] (EJB default - 8) DEBUG IMAPS: IMAPStore cleanup done
Setting mail.imaps.partialfetch=false the problem persists but yields a little different ouput with some message.
2023-01-05 17:15:11,663 INFO [stdout] (EJB default - 6) BUU52 FETCH 47 (BODY[1])
2023-01-05 17:17:11,859 ERROR [stderr] (EJB default - 6) javax.mail.FolderClosedException: * BYE JavaMail Exception: java.io.IOException: Connection dropped by server?
2023-01-05 17:17:11,874 ERROR [stderr] (EJB default - 6) at javax.mail.api#1.6.2//javax.mail.internet.MimeBodyPart.getContent(MimeBodyPart.java:685)
This tightens my assumption with the timeout on the server side. The question is: What is causing the freeze and how to resolve it? Any hints or suggestions to inspect this in more detail are welcome.