sending mails does not always work if user has more than 1 account in Outlook profile

Question

I don't know if it is Outlook related or Exchange related, but I guess it is something in the profile of the user, so Outlook. We have litteraly this issue for months but it is not always "active". Exchange 2016 cu23 (latest updates ok) on-prem + Outlook client 2016 (latest updates). UserA has an AD account with a mailbox userA@domain.com. UserA is member of an address listA and address group policyA. UserA has also other mailboxes on the same on-prem Exchange server. So userB with userB@domainB.com. This user is using address listB and address group policyB. Configured with other account. So UserA has 1 Outlook profile with different mailboxes. Mostly with mailboxes and own credentials. I don't want to make it difficult but that user has also different shared mailboxes. All with send-as permissions. The ISSUE.. User can send messages with own mailbox(mail address) and inbox becomes empty. when he sends message from another mailbox sometimes the outbox does not relieve the message. It stays in outbox, in italics (so the process of sending is still going further). Maybe important to say that it is on a Citrix environment. And the things I already tried are countless from new Outlook profiles, other Citrix server, .... Network possibilities, Active directory checks, dns checks, .... Sometimes there are days that it goes flawless, sometimes it just doesn't work (sending e-mails stays in outbox). I really don't know where to look anymore. Although nowadays I think the system remembers the passwords. For the mailboxes they are all different. I think when the user wants to send from another mail address the system uses the credentials from the primary mailbox and that is why it is staying in the outbox. The primary mailbox has no send-as permissions, because I added the extra mailbox with the credentials of the extra account which is connected to that specific mailbox. I know there is no quick solution, but maybe someone has more experience with it and can point me to a better direction. Thanks for your help. Kurt