Outlook does not recognize one or more names

We have become aware of users running Outlook/Office 365 getting this error over the past 2 weeks that never were before. In the past this error usually was a result of (1) an illegal email address, or (2) a corruption in Outlook such as a corrupt Auto-Complete list, (3) corrupted address book, or (4) corrupted Outlook profile.  Illegal addresses include ones that include an invalid format or illegal characters such as a space, semicolon, colon, or other special characters not allowed in an address, leaving off the .com or .net suffix. etc.  Historically this has been the # 1 cause of these errors.  And as far as the corruption problems, there is also a lot of information on the internet searches about correcting those in Outlook for cases where that has been the cause. 

However, these common issues seen in the past do not appear to be the cause of this increase in these errors recently.  After much research, it appears that Microsoft has updated Classic Outlook 365 users and created/imposed a new problem where if another program (such as Answers) tries to submit an email to it to be sent and the user has more than 1 email account configured in their Outlook profile, it will no longer send out the email and will throw this error.  From our research it appears that Outlook 365 has been changed to no longer assume that it should send using the default email account nor will it prompt for the account to use on automated emails and thus fails.  Unfortunately we did not receive any advance warning of this change as we do review the update notifications from Microsoft for ones that might potentially affect us or our customers.  Some searches we found are referring to this as a new "bug" in Outlook while others state it was done intentionally by Microsoft to reduce the chance of "spoofing" email address senders.  Regardless it has left Classic Outlook 365 users with multiple email accounts setup unable to email out of Answers.  This update (or "breaking") was most likely included with the April updates Microsoft rolled out the starting the 2nd Tuesday 04/14/26.

We have attempted multiple ideas to try to work around this problem even for the short-term with no luck yet.  However work on that continues.  Microsoft is really pushing for internet communications with their cloud services as opposed to directly with their desktop Classic Outlook app, so emailing for 365 users may change in the future due to this but if needed we will update everyone interested on that in the future. 

For now the best work-around regrettably is to remove additional email accounts from Outlook.  Keep in mind that if these accounts are IMAP/Exchange/365 type accounts, that removing them is just removing them from Outlook and the emails should still be on cloud based servers.  But you do want to verify this if you have other non-365/non-Exchange type email accounts configured in Outlook before removing them (such as older POP3 type email accounts). Reducing Outlook down to only the default email account has resolved this problem everywhere we have tested it.  To do this, get into Outlook and click on File-->Account Settings button-->Account Settings.  The default account is the one with the check-mark beside it.

Now obviously if you remove them you may very well still need to read those emails or reply or send new emails from those alternate emails.  If that is the case there are options available.  What most users have done thus far is to access those in a web browser using Outlook on the Web.  

Another option is to add these alternate accounts into the "New Outlook" (which does not allow other programs to send thru it at all as some of you have already encountered if you toggled to it and and then got an error and had to toggle back to the Classic Outlook).  The 2 Outlooks can now be ran along side either other, as this did not used to be the case.  Even their icons are slightly different, subtle, but different if users prefer an email app or program as opposed to a web version.

Another option that has been brought to our attention that may help some users if (a) your other email account(s) are also Microsoft 365 accounts and (b) all under the same 365 tenant.  This option unfortunately does not apply to other types of email accounts such as email accounts from Gmail, outlook.com, AOL, local ISP providers, etc. since these are usually IMAP, POP3, and SMTP type accounts.  But if they are both/all under the same 365 tenant account, you can contact whoever manages your Office 365 email and have them sign into your 365 Admin portal.  Once in click on Users-->Active Users.  Then click on the name of the secondary email account that you had to remove (or plan on removing) from Classic Outlook as suggested above.  When the properties window opens, click on "Mail" along the top. Next click on "Read and manage permissions" link.  In the next window, click on "+ Add permissions".  You will then see a list of all your users.  Check the box on the user(s) that you want to allow to also read this mailbox and then click the [Add] button at the bottom of the window.  You should then see the selected users listed below showing they were added.  Click the back arrow to go back to the previous window.  Next click on the "Send as permissions" link.  Repeat the same process here to allow another user(s) to send emails from this mailbox.  You do not need to edit the "Send on behalf of permissions".  Once this is done and you allow some time for 365 to update throughout it's system, you can remove this secondary account from your Outlook, exit, and restart it and the secondary email will appear as an alternate account just like before, but without it having to be specifically added as an email under File-->Account Settings-->Account Settings.  For example, a user "john" has his own email address within your company 's domain and that is his primary/default.  But he also monitors and sometimes wants to use a generic "sales" email.  In this case you would remove the "sales" email from his Outlook.  You would configure 365 to allow John "Read" and "Send as" permissions to the sales email.  Then when the user restarts his Outlook, he will still have his own email as the primary and default, but will also see the "sales" email account show up automatically because of how 365 gave him permissions to that email as an alternate.  If you have any questions on concerns about this, be sure to discuss it with whoever manages your Microsoft/Office 365 email accounts before proceeding.

Granted it is more convenient to have all your emails in one place(in your main Outlook), but at least these ideas provides a work-around to where you can continue to email out of Answers thru Outlook, while still providing an alternative way to access other email accounts.

And again, as stated above, this is a very new and evolving problem and research/testing on it continues and we will do our best to keep everyone updated with what we find out for moving forward.