I have a user mailbox which is around 24GB. This mailbox is sorted into various folders and subfolders and
has all of its sorted mail under INBOX. (whereas the normal folders like Darafts, Trash, etc. are in the
root folder).

So it's something like:

INBOX.FolderA.FolderA1.FolderA11
INBOX.FolderA.FolderA1.FolderA12
...
INBOX.FolderB.FolderB1.FolderB11
INBOX.FolderB.FolderB1.FolderB12
...

FolderA + FolderB are in total around 20GB with all subfolders that they contain.

Due to amount of mail that needs to be synced upon initial sync of an EAS mailbox, sogo-activesync crashes
on initial sync (i.e. with Outlook Mail client) when SOGoMaximumSyncResponseSize isn't set and therefore seems to be "unlimited" (my guess is that Outlook doesn't propose a value for it so it stays at 0, but I have no idea about EAS protocol):

#0 memcpy_avx_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:769
0000001 0x00007f404b9e4798 in memcpy (len=2675045, src=<optimized out>, dest=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/string_fortified.h:29
0000002 -[GSMutableString replaceCharactersInRange:withString:] (self=0x55d6fd624560, _cmd=<optimized out>, aRange=..., aString=0x55d77e12bc90) at ./Source/GSString.m:5228
0000003 0x00007f404734c43a in -[SOGoActiveSyncDispatcher(Sync) processSyncCollection:inBuffer:changeDetected:maxSyncResponseSize:] (self=0x55d6e17fb270,
_cmd=0x7f404737a130 <_OBJC_SELECTOR_TABLE+2640>, theDocumentElement=0x55d6e177bc70, theBuffer=0x55d6fd624560, changeDetected=0x7ffde5ed1abf "\001`\033\355\345\375\177",
theMaxSyncResponseSize=0) at SOGoActiveSyncDispatcher+Sync.m:2438
0000004 0x00007f404734cefe in -[SOGoActiveSyncDispatcher(Sync) processSync:inResponse:] (self=0x55d6e17fb270, _cmd=0x55d6e13fd450, theDocumentElement=0x55d6e0fdfe10, theResponse=0x55d6e13e4500)
at SOGoActiveSyncDispatcher+Sync.m:2662
0000005 0x00007f404733c013 in -[SOGoActiveSyncDispatcher dispatchRequest:inResponse:context:] (self=0x55d6e17fb270, _cmd=0x7f40473cfd20 <_OBJC_SELECTOR_TABLE+128>, theRequest=0x55d6e1bd40d0,
theResponse=0x55d6e13e4500, theContext=0x55d6e114e270) at SOGoActiveSyncDispatcher.m:4442
0000006 0x00007f40473c5c2a in -[SOGoMicrosoftActiveSyncActions microsoftServerActiveSyncAction] (self=0x55d6e1814af0, _cmd=0x55d6e13fd470) at SOGoMicrosoftActiveSyncActions.m:59
0000007 0x00007f404c34b295 in -[WODirectAction performActionNamed:] (self=0x55d6e1814af0, _cmd=0x7f404c522ca0 <_OBJC_SELECTOR_TABLE+928>, _actionName=0x55d6e0f5b220) at WODirectAction.m:97
0000008 0x00007f404c3e51a6 in -[SoActionInvocation callOnObject:withPositionalParametersWhenNotNil:inContext:] (self=0x55d6e17ec500, _cmd=0x7f404c522cd0 <_OBJC_SELECTOR_TABLE+976>,
_client=0x55d6e12968d0, _positionalArgs=0x0, _ctx=0x55d6e114e270) at SoActionInvocation.m:300
0000009 0x00007f404c3e52ef in -[SoActionInvocation callOnObject:inContext:] (self=0x55d6e17ec500, _cmd=0x7f404c51c9a0 <_OBJC_SELECTOR_TABLE+672>, _client=0x55d6e12968d0, _ctx=0x55d6e114e270)
at SoActionInvocation.m:318
0000010 0x00007f404c3df069 in -[SoObjectMethodDispatcher dispatchInContext:] (self=0x55d6e180e3e0, _cmd=0x7f404c51ee40 <_OBJC_SELECTOR_TABLE+1536>, _ctx=0x55d6e114e270)
at SoObjectMethodDispatcher.m:192
0000011 0x00007f404c3e1804 in -[SoObjectRequestHandler handleRequest:inContext:session:application:] (self=0x55d6e12ab850, _cmd=0x7f404c4a8c10 <_OBJC_SELECTOR_TABLE+848>, _rq=0x55d6e1bd40d0,
_ctx=0x55d6e114e270, _sn=0x0, app=0x55d6e12968d0) at SoObjectRequestHandler.m:584
0000012 0x00007f404c35e716 in -[WORequestHandler handleRequest:] (self=0x55d6e12ab850, _cmd=0x7f404c471190 <_OBJC_SELECTOR_TABLE+1616>, _request=0x55d6e1bd40d0) at WORequestHandler.m:240
0000013 0x00007f404c3198d6 in -[WOCoreApplication dispatchRequest:usingHandler:] (self=0x55d6e12968d0, _cmd=0x7f404c4711e0 <_OBJC_SELECTOR_TABLE+1696>, _request=0x55d6e1bd40d0, handler=0x55d6e12ab850)
at WOCoreApplication.m:712
0000014 0x00007f404c319c41 in -[WOCoreApplication dispatchRequest:] (self=0x55d6e12968d0, _cmd=0x55d6e01ea6a0 <_OBJC_SELECTOR_TABLE+1664>, _request=0x55d6e1bd40d0) at WOCoreApplication.m:752
0000015 0x000055d6e01e08fc in -[SOGo dispatchRequest:] (self=0x55d6e12968d0, _cmd=0x7f404c50ed00 <_OBJC_SELECTOR_TABLE+1760>, _request=0x55d6e1bd40d0) at SOGo.m:584
0000016 0x00007f404c3ce226 in -[WOHttpTransaction _run] (self=0x55d6e0fc2860, _cmd=0x7f404c50ed30 <_OBJC_SELECTOR_TABLE+1808>) at WOHttpTransaction.m:566
0000017 0x00007f404c3ce5ec in -[WOHttpTransaction run] (self=0x55d6e0fc2860, _cmd=0x7f404c50b250 <_OBJC_SELECTOR_TABLE+1168>) at WOHttpTransaction.m:619
0000018 0x00007f404c3c9cd3 in -[WOHttpAdaptor runConnection:] (self=0x55d6e1383270, _cmd=0x7f404c50b2f0 <_OBJC_SELECTOR_TABLE+1328>, _socket=0x55d6e118bec0) at WOHttpAdaptor.m:373
0000019 0x00007f404c3c9f2a in -[WOHttpAdaptor _handleAcceptedConnection:] (self=0x55d6e1383270, _cmd=0x7f404c50b300 <_OBJC_SELECTOR_TABLE+1344>, _connection=0x55d6e118bec0) at WOHttpAdaptor.m:407
0000020 0x00007f404c3ca3a3 in -[WOHttpAdaptor _handleConnection:] (self=0x55d6e1383270, _cmd=0x7f404c50b3a0 <_OBJC_SELECTOR_TABLE+1504>, connection=0x55d6e118bec0) at WOHttpAdaptor.m:466
0000021 0x00007f404c3ca6c1 in -[WOHttpAdaptor acceptControlMessage:] (self=0x55d6e1383270, _cmd=0x7f404c50b0f0 <_OBJC_SELECTOR_TABLE+816>, aNotification=0x55d6e167cfa0) at WOHttpAdaptor.m:505
0000022 0x00007f404bac6ccb in -[NSNotificationCenter _postAndRelease:] (self=0x55d6e0fb6c20, _cmd=<optimized out>, notification=0x55d6e167cfa0) at ./Source/NSNotificationCenter.m:1198
0000023 0x00007f404c01cc6e in -[NSObject(FileObjectWatcher) receivedEvent:type:extra:forMode:] (self=0x55d6e1386100, _cmd=0x7f404be8fa30 <_OBJC_SELECTOR_TABLE+304>, _fdData=0x2c, _type=ET_RDESC,
_extra=0x2c, _mode=0x7f404be0a210 <_OBJC_INSTANCE_2>) at NSRunLoop+FileObjects.m:58
0000024 0x00007f404bbddeb6 in -[GSRunLoopCtxt pollUntil:within:] (self=<optimized out>, _cmd=0x7f404be093e0 <_OBJC_SELECTOR_TABLE+1184>, milliseconds=<optimized out>, contexts=0x55d6e1278f60)
at ./Source/unix/GSRunLoopCtxt.m:600
0000025 0x00007f404bb11674 in -[NSRunLoop acceptInputForMode:beforeDate:] (self=0x55d6e118f580, _cmd=0x7f404be09410 <_OBJC_SELECTOR_TABLE+1232>, mode=0x7f404be0a210 <_OBJC_INSTANCE_2>,
limit_date=0x55d6e13aa6e0) at ./Source/NSRunLoop.m:1254
0000026 0x00007f404bb11340 in -[NSRunLoop runMode:beforeDate:] (self=<optimized out>, _cmd=<optimized out>, mode=0x7f404be0a210 <_OBJC_INSTANCE_2>, date=<optimized out>) at ./Source/NSRunLoop.m:1334
0000027 0x00007f404c319045 in -[WOCoreApplication run] (self=0x55d6e12968d0, _cmd=0x55d6e01ea400 <_OBJC_SELECTOR_TABLE+992>) at WOCoreApplication.m:584
0000028 0x000055d6e01df945 in -[SOGo run] (self=0x55d6e12968d0, _cmd=0x7f404c4b1630 <_OBJC_SELECTOR_TABLE+624>) at SOGo.m:337
0000029 0x00007f404c3686bb in -[WOWatchDog _runChildWithControlSocket:] (self=0x55d6e11b40c0, _cmd=0x7f404c4b16d0 <_OBJC_SELECTOR_TABLE+784>, controlSocket=0x55d6e1386100)
at WOWatchDogApplicationMain.m:523
0000030 0x00007f404c368bc4 in -[WOWatchDog _spawnChild:] (self=0x55d6e11b40c0, _cmd=0x7f404c4b17a0 <_OBJC_SELECTOR_TABLE+992>, child=0x55d6e12b3fc0) at WOWatchDogApplicationMain.m:600
0000031 0x00007f404c369377 in -[WOWatchDog _ensureChildren] (self=0x55d6e11b40c0, _cmd=0x7f404c4b1960 <_OBJC_SELECTOR_TABLE+1440>) at WOWatchDogApplicationMain.m:690
0000032 0x00007f404c36a5a0 in -[WOWatchDog run:argc:argv:] (self=0x55d6e11b40c0, _cmd=0x7f404c4b1a70 <_OBJC_SELECTOR_TABLE+1712>, newAppName=0x55d6e01e81a0 <_OBJC_INSTANCE_3.1>, newArgC=7,
newArgV=0x7ffde5ed4c58) at WOWatchDogApplicationMain.m:942
0000033 0x00007f404c36adca in WOWatchDogApplicationMain (appName=0x55d6e01e81a0 <_OBJC_INSTANCE_3.1>, argc=7, argv=0x7ffde5ed4c58) at WOWatchDogApplicationMain.m:1051
0000034 0x000055d6e01de329 in main (argc=7, argv=0x7ffde5ed4c58, env=0x7ffde5ed4c98) at sogod.m:51

It turns out that this is most likely due to the fact that an NSMutableString memory
buffer seems to be limited to 32bit (4GB) and appending all the stuff of the inbox
seems to overflow it:

(gdb) frame 2

0000002 -[GSMutableString replaceCharactersInRange:withString:] (self=0x55d6fd624560, _cmd=<optimized out>, aRange=..., aString=0x55d77e12bc90) at ./Source/GSString.m:5228
5228 in ./Source/GSString.m
(gdb) print aRange

$7 = {location = 4292575740, length = 0}

Context:

[theBuffer appendString: changeBuffer];

->

-(void)appendString:(NSString *)string {
NSRange range={[self length],0};

[self replaceCharactersInRange:range withString:string];
}

theBuffer contains the first mail of the INBOX on its start and changeBuffer contains
the current mail. When adding size of changeBuffer to location above, it exceeds
0xFFFFFFFF, which in my opinion is the cause for the crash.

So maybe it is a good idea to set a sane upper limit on the SOGoMaximumSyncResponseSize when not configured to at least prevent a crash from happening, even though it can be seen as misconfiguration by the user to not set it at all in config.