Redmine

We experience problems with email notifications. Most of the emails arrive normally, but some of them don't. There are various errors in production.log:

[ActiveJob] [ActionMailer::DeliveryJob] [9e844fb0-ec90-4d15-9da9-727ce496315f] Email delivery error: 421 4.4.2 bulk.smtp.cz Error: timeout exceeded
...
[ActiveJob] [ActionMailer::DeliveryJob] [943897ea-4eaa-4cf3-a5f5-e74fc7655f04] Email delivery error: end of file reached
...
[ActiveJob] [ActionMailer::DeliveryJob] [2beb2fdc-44c1-4fb4-8d95-50b5bcbc0788] Email delivery error: execution expired
...
[ActiveJob] [ActionMailer::DeliveryJob] [36650f3b-628e-41f0-ae60-eca9ecc3e9f5] Email delivery error: Net::OpenTimeout
...
[ActiveJob] [ActionMailer::DeliveryJob] [6262d176-edae-45e3-9621-34df77ee2845] Email delivery error: SSL_connect SYSCALL returned=5 errno=0 state=SSLv3/TLS write client hello

These errors repeat from time to time.

I've tried:
- 3 different SMTP servers, even smtp.mandrillapp.com
- ports 465/587/25
No matter what I set, the situation is the same, approximatelly the same amount of emails don't arrive.

Something is wrong and I have no idea what to do with it.

Please don't you have any idea what could I try?

 
 
OS: Debian 11 with all updates

ruby bin/about:

sh: 1: hg: not found
sh: 1: cvs: not found
sh: 1: bzr: not found
Environment:
  Redmine version                4.2.5.stable.21531
  Ruby version                   2.7.4-p191 (2021-07-07) [x86_64-linux-gnu]
  Rails version                  5.2.7
  Environment                    production
  Database adapter               Mysql2
  Mailer queue                   ActiveJob::QueueAdapters::AsyncAdapter
  Mailer delivery                smtp
SCM:
  Subversion                     1.14.1
  Git                            2.30.2
  Filesystem                     
Redmine plugins:
  clipboard_image_paste          1.13
  redmine_agile                  1.6.2
  redmine_checklists             3.1.20
  redmine_custom_css             0.1.7
  redmine_custom_js              0.1.1
  redmine_hide_sidebar           4.0.0
  redmine_issue_dynamic_edit     0.7.2
  redmine_lightbox2              0.5.1
  redmine_per_project_formatting 0.1.0
  redmine_theme_changer          0.4.0
  redmineup_tags                 2.0.11