send mail not work after upgrade to V5.0.0

When you send email manually, do you selected receivers and/or enter specific email address ?
And are you sure your received have an email address ?

If no receiver can be defined, you will get message "Not able to send mail to '', Contact your administrator".
But this is not a technical issue, just a warning that no email is sent.

Note that this can also happen if you select a receiver.
For instance, if you select "Responsible" as receiver, but no resposible is defined for the item, no email can be sent.
The same if responsible is set but the corresponding resource has no eamil defined.

For information, I change type of message so that in latest described cases, non error is generated, just a warning.

... and email is working when generating alerts ?
... very strange.

Could you check email for special characters, others than @.
Maybe you can also try and send email to single address (your own for instance)

What is strange is that some emails are correctly sent.
Not all fail.
So I would bet on SMTP server or more possibly some network issue (possibly some proxy playing with connections)