Sending Segment Emails in "immediate" mode results in partial send

Your software
My Mautic version is: 2.82
My PHP version is: 5.6.37
My Database type and version is:

Your problem
My problem is: Have email config set to send “immediately”. If i send more than around 1100-1200 (not sure when it quits exactly) out of 2000 emails, the status bar which usually take a while to update after every 100 emails sent, goes quickly to 1300,1400,1500… 2000 and I know it is not sending emails anymore. I’m on my own VPS and have not put any restrictions on emails sent. Ideas? Alternately, when I send a segment email and I knew that the queue with the correct cron job would work, I could switch to this for these email jobs. It really bugs me cuz now I have no idea who didn’t get the email.

These errors are showing in the log:
[2022-03-18 16:13:41] mautic.ERROR: [MAIL ERROR] Expected response code 250 but got code “”, with message “” Log data: >> MAIL FROM: << !! Expected response code 250 but got code “”, with message “” (code: 0) (send)
****could still be trying to send some mails that weren’t deliverable yesterday?

Steps I have tried to fix the problem: not sure what to do here…

Mautic 2.X is no longer supported. There are some great features in version 3.X and 4.X and more secure. Have you considered upgrading your 2.X installs to a current version?