Problem with mail notification

[Admin_SOFTEX]

Good morning,
I am new to Bamboo and have been testing the system for over a month.
My problem is that occasionally (often) notification mails don't arrive. Both on the build side (less rarely) and on the deploy side.

By starting the server in console I can read this message:

2020-03-11 18:18:59,162 WARN [mail-queue-processor] [SMTPMailServerWithEncryptedPassword] An error has occurred whilst closing the connection to the outgoing mail server: Bamboo. This could be caused by a time-out on closing this connection, increase the timeout in smtp server configuration. Alternatively, set the 'mail.smtp.quitwait' (or 'mail.smtps.quitwait' for ssl) system property to false.
javax.mail.MessagingException: Can't send command to SMTP host;
nested exception is:
java.net.SocketException: Connection closed by remote host
at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:2259)
at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:2246)
at com.sun.mail.smtp.SMTPTransport.close(SMTPTransport.java:1287)
at com.atlassian.mail.server.impl.SMTPMailServerImpl.sendWithMessageId(SMTPMailServerImpl.java:195)
at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:162)
at com.atlassian.bamboo.mail.EmailService.sendEmailSynchronously(EmailService.java:115)
at com.atlassian.bamboo.mail.EmailService$MailQueueProcessor.run(EmailService.java:67)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.net.SocketException: Connection closed by remote host
at sun.security.ssl.SSLSocketImpl.checkWrite(SSLSocketImpl.java:1547)
at sun.security.ssl.AppOutputStream.write(AppOutputStream.java:71)
at com.sun.mail.util.TraceOutputStream.write(TraceOutputStream.java:128)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)
at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:2257)
... 7 more
2020-03-11 18:18:59,162 WARN [mail-queue-processor] [EmailService] Uncaught exception while processing mail queue
com.atlassian.mail.MailException: com.sun.mail.smtp.SMTPSendFailedException: 552 5.2.0 D5KR2200G2ewmCh015KR5d spam detected

at com.atlassian.mail.server.impl.SMTPMailServerImpl.sendWithMessageId(SMTPMailServerImpl.java:225)
at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:162)
at com.atlassian.bamboo.mail.EmailService.sendEmailSynchronously(EmailService.java:115)
at com.atlassian.bamboo.mail.EmailService$MailQueueProcessor.run(EmailService.java:67)
at java.lang.Thread.run(Thread.java:748)
Caused by: com.sun.mail.smtp.SMTPSendFailedException: 552 5.2.0 D5KR2200G2ewmCh015KR5d spam detected

at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:2202)
at com.sun.mail.smtp.SMTPTransport.finishData(SMTPTransport.java:1980)
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1197)
at com.atlassian.mail.server.impl.SMTPMailServerImpl.sendMimeMessage(SMTPMailServerImpl.java:245)
at com.atlassian.mail.server.impl.SMTPMailServerImpl.sendWithMessageId(SMTPMailServerImpl.java:187)