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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 18.0.2): [Nextcloud 19.0.1
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18.04
PHP version (eg, 7.1): 7.3.17
The issue you are facing:
When a Nextcloud user - who’s email address contains the characters “+work” creates a calendar event, and then invites 1 or more email address, the invited email address does not receive the email invite.
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- create calendar event in Nextcloud’s users calendar
- add email address to attendees
- click save
(I have also replicated this behaviour on a commercially hosted Nextcloud instance.)
Expected behaviour:
Within a few seconds/minutes invited attendee gets email invite to event.
Actual behaviour:
No invite is received by invited party.
The output of your Nextcloud log in Admin > Logging:
[lib] Error: ResourceLocator can not find a web root (root: /app/data/apps/notifications, file: css/notifications/b1d7-c532-styles.css?v=b92d206521717ac032f8aa58d3c7ff2f, webRoot: , throw: true)
GET /apps/calendar/timeGridWeek/now/new/sidebar/0/1596814200/1596816000
from XXXXXXXX by bernat at 2020-08-07T13:59:37+00:00
[lib] Error: ResourceLocator can not find a web root (root: /app/data/apps/notifications, file: css/notifications/b1d7-c532-styles.css?v=b92d206521717ac032f8aa58d3c7ff2f, webRoot: , throw: true)
GET /apps/calendar/timeGridWeek/now/new/sidebar/0/1596814200/null
from XXXXXXXX by bernat at 2020-08-07T13:59:51+00:00
[PHP] Error: session_start(): A session had already been started - ignoring at /app/code/lib/private/Session/Internal.php#209
PROPFIND /remote.php/dav/calendars/bernat/
from XXXXXXX at 2020-08-07T14:00:38+00:00
I am happy to provide more logs and debug further with help.
I posted this here as I felt the issue was more with Nextcloud “core” functionality than with calendar. If the issue is with the calendar app and not the core Nextcloud, then I’m happy to move it to another category.