Regarding your question about possiblity of using single outgoing mail server for multiple users with their own email id you should configure SMTP Relay server at Office 365 as described this (https://ventor.tech/guides/how-to-configure-emails-to-work-with-office-365-and-odoo/#1) article, that it will work as you want.

Please check the error messages you get back from your SMTP provider and make sure your provider allows to send from multiple addresses. Also read the documentation carefully:

https://www.odoo.com/documentation/14.0/applications/productivity/discuss/advanced/email_servers.html#

To make Microsoft Email work properly, you have to make sure you have setup the relay capability of your SMTP server as described by Microsoft, a second requirement is to create a catchall account for the incoming mail (which is also not easy with MS). I have found a clever way to achieve this goal (it is in German only):

https://www.langlitz-it.de/?p=2320

Mail Delivery Failed Mail delivery failed via SMTP server 'None'. SMTPDataError: 554 5.2.252 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message Cannot submit message. 0.35250:D7250000, 1.36674:0A000000, 1.61250:00000000, 1.45378:02000000, 1.44866:B5380000, 1.36674:0E000000, 1.61250:00000000, 1.45378:BA380000, 1.44866:0C010000, 16.55847:441D0000, 17.43559:0000000024020000000000000000000000000000, 20.52176:140F69910600101000000000, 20.50032:140F69917617101033000000, 0.35180:BE000000, 255.23226:00000000, 255.27962:0A000000, 255.27962:0E000000, 255.31418:140F6991, 0.35250:00000000, 1.36674:0A000000, 1.61250:00000000, 1.45378:02000000, 1.44866:4E000000, 1.36674:32000000, 1.61250:00000000, 1.45378:53000000, 1.44866:01000000, 16.55847:BA000000, 17.43559:0000000088030000000000000000000000000000, 20.52176:140F69910600101047000000, 20.50032:140F69917617F01F36000000, 0.35180:02000000, 255.23226:A4000000, 255.27962:0A000000, 255.27962:32000000, 255.17082:DC040000, 0.27745:4F000000, 4.21921:DC040000, 255.27962:FA000000, 255.1494:0F010480, 0.38698:46000000, 1.41134:46000000, 0.37692:86000000, 0.37948:86000000, 5.33852:00000000534D545000000000, 7.36354:010000000000010986000000, 4.56248:DC040000, 7.40748:010000000000010B14A9CDD8, 7.57132:0000000000000000B6EA542A, 1.63016:32000000, 4.39640:DC040000, 8.45434:9ECF1575FF3CAB4BB6EA542AA5CA10A130363A44, 1.46798:04000000, 5.10786:0000000031352E32302E343435372E3030363A444241505230374D42363835343A65313662303636622D646237342D343862352D396335612D3737333034653638656436383A31353738383000FA101002000000, 7.51330:E90CD721D663D90800000000, 0.39570:12000000, 1.55954:0A000000, 0.49266:00000000, 1.33010:0A000000, 2.54258:00000000, 0.40002:7A000000, 1.56562:00000000, 1.64146:32000000, 1.33010:32000000, 2.54258:DC040000, 255.1750:13000000, 255.31418:00000000, 0.22753:18000000, 255.21817:DC040000, 0.64418:00000000, 4.39842:DC040000, 0.41586:00000000, 4.60547:DC040000, 0.21966:0F010480, 4.30158:DC040000 [Hostname=DBAPR07MB6854.eurprd07.prod.outlook.com]