The nodemailer connection is forcibly closed in Heroku after 5 min - node.js

Question

I'm using nodemailer library as an SMTP client to send emails outside. I'm also using the option for connection pooling that keeps socket open in the air for 10 min. After the 10 min of inactivity the socket is closing.
smtpClient = nodemailer.createTransport({... pool:true ...});
This is how it looks like when the connection is closing the regular way after the timeout event:
{ component: 'smtp-connection', sid: 'cHH7rJsPKGc' } Timeout
{ component: 'smtp-pool', err: { Error: Timeout
at SMTPConnection._onTimeout (/app/node_modules/nodemailer/lib/smtp-connection/index.js:847:30)
at TLSSocket.SMTPConnection._onSocketTimeout (/app/node_modules/nodemailer/lib/smtp-connection/index.js:199:44)
at TLSSocket.emit (events.js:314:20)
at TLSSocket.EventEmitter.emit (domain.js:483:12)
at TLSSocket.Socket._onTimeout (net.js:484:8)
at listOnTimeout (internal/timers.js:554:17)
at processTimers (internal/timers.js:497:7)
code: 'ETIMEDOUT',
command: 'CONN' }, tnx: 'pool', cid: 1 } Pool Error for #%s: %s
{ component: 'smtp-connection', sid: 'cHH7rJsPKGc', tnx: 'smtp' } Closing connection to the server using "%s"
{ component: 'smtp-pool', tnx: 'connection', cid: 1, action: 'closed' } Connection #%s was closed
When running it on Heroku, it doesn't reach the 10 min but closes after the 4-5 min with an error:
{ component: 'smtp-pool', err: { Error: Unexpected Response: 451 4.7.0 Timeout waiting for client input [xxx.prod.protection.outlook.com]
at SMTPConnection._processResponse (/app/node_modules/nodemailer/lib/smtp-connection/index.js:945:34)
at SMTPConnection._onData (/app/node_modules/nodemailer/lib/smtp-connection/index.js:749:14)
at TLSSocket.SMTPConnection._onSocketData (/app/node_modules/nodemailer/lib/smtp-connection/index.js:195:44)
at TLSSocket.emit (events.js:314:20)
at TLSSocket.EventEmitter.emit (domain.js:483:12)
at addChunk (_stream_readable.js:298:12)
at readableAddChunk (_stream_readable.js:273:9)
at TLSSocket.Readable.push (_stream_readable.js:214:10)
at TLSWrap.onStreamRead (internal/stream_base_commons.js:188:23)
code: 'EPROTOCOL',
response: '451 4.7.0 Timeout waiting for client input [xxx.prod.protection.outlook.com]',
responseCode: 451,
command: 'CONN' }, tnx: 'pool', cid: 2 } Pool Error for #%s: %s
{ component: 'smtp-pool', tnx: 'connection', cid: 2, action: 'closed' } Connection #%s was closed
{ component: 'smtp-connection', sid: 'Wabdmex9scE', tnx: 'network' } Connection closed
Just to remind, the nodemailer uses Node.js tls.connect() to open a socket:
this._socket = tls.connect(opts, () => {
this._socket.setKeepAlive(true);
this._socket.setTimeout(10 * 60 * 1000);
});
Node.js version: "12.15.0"-"12.19.0"
nodemailer version: "^6.1.1"